fix: macOS build broken on nixos-unstable

This is definitely a bug in rapidcheck but also rapidcheck is
unmaintained (author vanished) so we can't really do anything about it
on that side.

lix> FAILED: tests/unit/liblixutil-test-support.dylib.p/libutil-support_tests_hash.cc.o
lix> clang++ -Itests/unit/liblixutil-test-support.dylib.p -Itests/unit -I../tests/unit -I../tests/unit/libutil-support -I. -I.. -Ilix/libutil -Ilix/libexpr -fdiagnostics-color=always -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST -Wall -Winvalid-pch -Wextra -std=c++23 -O2 -g -fpch-instantiate-templates -include config.h -Wno-unused-parameter -Wno-deprecated-declarations -Wno-missing-field-initializers -Wimplicit-fallthrough -Werror=switch -Werror=switch-enum -Werror=unused-result -Wdeprecated-copy -Wignored-qualifiers -Werror=suggest-override -fsanitize=signed-integer-overflow -fsanitize-undefined-trap-on-error -ffile-prefix-map=../lix=lix -MD -MQ tests/unit/liblixutil-test-support.dylib.p/libutil-support_tests_hash.cc.o -MF tests/unit/liblixutil-test-support.dylib.p/libutil-support_tests_hash.cc.o.d -o tests/unit/liblixutil-test-support.dylib.p/libutil-support_tests_hash.cc.o -c ../tests/unit/libutil-support/tests/hash.cc
lix> In file included from ../tests/unit/libutil-support/tests/hash.cc:3:
lix> In file included from /nix/store/k8b7wc0lkr3lqxybzv458ijddpqnzlgw-rapidcheck-0-unstable-2023-12-14-dev/include/rapidcheck.h:18:
lix> In file included from /nix/store/k8b7wc0lkr3lqxybzv458ijddpqnzlgw-rapidcheck-0-unstable-2023-12-14-dev/include/rapidcheck/Gen.h:74:
lix> /nix/store/k8b7wc0lkr3lqxybzv458ijddpqnzlgw-rapidcheck-0-unstable-2023-12-14-dev/include/rapidcheck/Gen.hpp:72:22: error: calling 'current_exception' with incomplete return type 'exception_ptr'
lix>    72 |     auto exception = std::current_exception();
lix>       |                      ^~~~~~~~~~~~~~~~~~~~~~~~
lix> /nix/store/0nhi47d5ip48wprxnava6vv973zzzndf-libcxx-19.1.5-dev/include/c++/v1/__exception/operations.h:37:41: note: 'current_exception' declared here
lix>    37 | _LIBCPP_EXPORTED_FROM_ABI exception_ptr current_exception() _NOEXCEPT;
lix>       |                                         ^
lix> /nix/store/0nhi47d5ip48wprxnava6vv973zzzndf-libcxx-19.1.5-dev/include/c++/v1/__exception/operations.h:35:33: note: forward declaration of 'std::exception_ptr'
lix>    35 | class _LIBCPP_EXPORTED_FROM_ABI exception_ptr;
lix>       |                                 ^
lix> 1 error generated.

Change-Id: Id5d0c4aa8143f2469c5a636d6773a2f5a25a0268

tests/unit/libutil-support/tests/hash.cc

@@ -1,5 +1,6 @@
-#include <regex>
-
+// required due to rapidcheck generating an incomplete-return-type error due to
+// missing include on libc++ on macOS
+#include <exception> // IWYU pragma: keep
 #include <rapidcheck.h>
 
 #include "hash.hh"