nix-expr: Expose nix_api_expr_internal.h intentionally

This is required for other bindings like nix-flake-c to hook into
nix-expr-c appropriately.
The `_internal` part should be a sufficient deterrent normally,
and it may also be useful for bindings that migrate from the C++
interface.

src/libexpr-c/meson.build

@@ -37,13 +37,11 @@ include_dirs = [include_directories('.')]
 
 headers = files(
   'nix_api_expr.h',
+  'nix_api_expr_internal.h',
   'nix_api_external.h',
   'nix_api_value.h',
 )
 
-# TODO move this header to libexpr, maybe don't use it in tests?
-headers += files('nix_api_expr_internal.h')
-
 subdir('nix-meson-build-support/export-all-symbols')
 subdir('nix-meson-build-support/windows-version')