base_project: Install PC files in the pkg_dir directory

Otherwise we are only able to add PC files. Now instead we are able
to provide PC files that replaces the ones already provided upstream.

gvsbuild/utils/base_project.py

@@ -409,7 +409,7 @@ def install_dir(self, src, dest=None):
 
     def install_pc_files(self, base_dir="pc-files"):
         """Install, setting dir & version, the .pc files."""
-        pkgconfig_dir = os.path.join(self.builder.gtk_dir, "lib", "pkgconfig")
+        pkgconfig_dir = os.path.join(self.pkg_dir, "lib", "pkgconfig")
         self.builder.make_dir(pkgconfig_dir)
 
         src_dir = os.path.join(self._get_working_dir(), base_dir)