Install nlohmann_json headers

These headers are included by the libexpr, libfetchers, libstore
and libutil headers.
Considering that these are vendored sources, Nix should expose them,
as it is not a good idea for reverse dependencies to rely on a
potentially different source that can go out of sync.

src/nlohmann/local.mk

@@ -0,0 +1,2 @@
+$(foreach i, $(wildcard src/nlohmann/*.hpp), \
+  $(eval $(call install-file-in, $(i), $(includedir)/nlohmann, 0644)))