Include the pkgconfig-specified gdesktop-enums.h

Instead of blindly hoping that `$INCLUDE` contains the parent directory
of `gsettings-desktop-schemas`.

Because `gsettings-desktop-schemas.pc` says:
```
Cflags: -I/SOME/DIRECTORY/gsettings-desktop-schemas
```
Which means to include the version that Meson has configured you need
to drop the directory prefix and only `#include <gdesktop-enums.h>`.

This fixes a build failure with local installs triggered by 775ec67a44
but it's also the right thing to do™.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1370

src/backends/meta-input-settings-private.h

@@ -22,7 +22,7 @@
 #ifndef META_INPUT_SETTINGS_PRIVATE_H
 #define META_INPUT_SETTINGS_PRIVATE_H
 
-#include <gsettings-desktop-schemas/gdesktop-enums.h>
+#include <gdesktop-enums.h>
 
 #ifdef HAVE_LIBWACOM
 #include <libwacom/libwacom.h>

src/meta/meta-background-actor.h

@@ -21,7 +21,7 @@
 #ifndef META_BACKGROUND_ACTOR_H
 #define META_BACKGROUND_ACTOR_H
 
-#include <gsettings-desktop-schemas/gdesktop-enums.h>
+#include <gdesktop-enums.h>
 
 #include "clutter/clutter.h"
 #include "meta/meta-background.h"

src/meta/meta-background-content.h

@@ -21,7 +21,7 @@
 #ifndef META_BACKGROUND_CONTENT_H
 #define META_BACKGROUND_CONTENT_H
 
-#include <gsettings-desktop-schemas/gdesktop-enums.h>
+#include <gdesktop-enums.h>
 
 #include "clutter/clutter.h"
 #include "meta/meta-background.h"

src/meta/meta-background.h

@@ -21,7 +21,7 @@
 #ifndef META_BACKGROUND_H
 #define META_BACKGROUND_H
 
-#include <gsettings-desktop-schemas/gdesktop-enums.h>
+#include <gdesktop-enums.h>
 
 #include "clutter/clutter.h"
 #include "meta/display.h"