cogl-pango/meson.build: Remove extraneous quoting

It wasn't necessary (see other instances of -DG_LOG_DOMAIN) and somewhere
along the line it was getting turned into forward slashes becoming a syntax
error:

```
/usr/include/glib-2.0/gobject/gobject.h:767: syntax error, unexpected '/' in
...
g_assertion_message (/"CoglPango/",
```

https://gitlab.gnome.org/GNOME/mutter/merge_requests/841

cogl/cogl-pango/meson.build

@@ -62,7 +62,7 @@ if have_introspection
     ],
     extra_args: introspection_args + [
       '-UCOGL_COMPILATION',
-      '-DG_LOG_DOMAIN=\"CoglPango\"',
+      '-DG_LOG_DOMAIN="CoglPango"',
     ],
     install_dir_gir: pkglibdir,
     install_dir_typelib: pkglibdir,