libvirt-glib: fixup build after glib update

I see only one instance of the warning where *equality* of strings
is tested and char signedness differs. That one seems perfectly safe.
```
../libvirt-gconfig/libvirt-gconfig-domain-capabilities-os.c: In function 'search_firmwares':
../libvirt-gconfig/libvirt-gconfig-domain-capabilities-os.c:70:26: warning: pointer targets in passing argument 1 of 'strcmp' differ in signedness [-Wpointer-sign]
70 | if (!g_str_equal(node->name, "enum"))
/nix/store/fmqcm52w0p5jmzdbjjskwjiwyj9kqic9-glib-2.74.1-dev/include/glib-2.0/glib/ghash.h:165:39: note: in definition of macro 'g_str_equal'
165 | #define g_str_equal(v1, v2) (strcmp ((v1), (v2)) == 0)
| ^~
```

pkgs/development/libraries/libvirt-glib/default.nix

···
 
   strictDeps = true;
 
+  # https://gitlab.com/libvirt/libvirt-glib/-/issues/4
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=pointer-sign" ];
+
   meta = with lib; {
     description = "Library for working with virtual machines";
     longDescription = ''