CI: install glib for the MSYS2 jobs to make pkgconf happy

Fixes a test when using pkgconf instead of pkg-config.

The .pc file in the pkgconfig-gen test requires it and pkgconf seems be
stricter here and fails to validate.

.github/workflows/msys2.yml

@@ -56,6 +56,7 @@ jobs:
             mercurial
             lcov
             mingw-w64-${{ matrix.MSYS2_ARCH }}-cmake
+            mingw-w64-${{ matrix.MSYS2_ARCH }}-glib2
             mingw-w64-${{ matrix.MSYS2_ARCH }}-libxml2
             mingw-w64-${{ matrix.MSYS2_ARCH }}-ninja
             mingw-w64-${{ matrix.MSYS2_ARCH }}-pkg-config