packaging: only install lix_doc for static builds

The stated rationale for installing lix_doc static library only applies to
static builds. Doing it even for dynamic builds is unnecessary and has lead to
the questionable practice of deleting the library in the nixpkgs packaging [1],
in turn contributing to breaking downstream static linkage.

[1] https://github.com/NixOS/nixpkgs/blob/bbb1dbe938ff2341e6618872671de7432c98078f/pkgs/tools/package-management/lix/common-lix.nix#L294-L296

Change-Id: I6a6a69642be04c6ee6976e324ab87abf1e112b90

lix/lix-doc/meson.build

@@ -13,7 +13,7 @@ lix_doc = static_library(
   # that to link with `-Wl,--whole-archive`, unless we also install this target.
   # `-Wl,--whole-archive` can cause some Problems when linking multiple nested
   # static libraries, so let's just install the damn thing.
-  install : true,
+  install : is_static,
 )
 
 liblix_doc = declare_dependency(