gpu: Don't include monitor manager header

Doing so pulls in too much, causing potential header include order
conflicts.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4192>

src/backends/meta-color-device.c

@@ -24,6 +24,7 @@
 
 #include <colord.h>
 
+#include "backends/meta-backend-private.h"
 #include "backends/meta-color-device.h"
 #include "backends/meta-color-manager-private.h"
 #include "backends/meta-color-profile.h"

src/backends/meta-color-manager.c

@@ -50,6 +50,7 @@
 #include "backends/meta-backend-types.h"
 #include "backends/meta-color-device.h"
 #include "backends/meta-color-store.h"
+#include "backends/meta-monitor-manager-private.h"
 #include "backends/meta-monitor.h"
 
 #include "meta-dbus-gsd-color.h"

src/backends/meta-crtc.c

@@ -20,6 +20,7 @@
 #include "backends/meta-crtc.h"
 
 #include "backends/meta-gpu.h"
+#include "meta/meta-backend.h"
 
 enum
 {

src/backends/meta-gpu.h

@@ -21,7 +21,7 @@
 
 #include <glib-object.h>
 
-#include "backends/meta-monitor-manager-private.h"
+#include "core/util-private.h"
 
 #define META_TYPE_GPU (meta_gpu_get_type ())
 META_EXPORT_TEST

src/backends/meta-output.c

@@ -21,6 +21,7 @@
 
 #include "backends/edid.h"
 #include "backends/meta-crtc.h"
+#include "backends/meta-monitor-manager-private.h"
 
 enum
 {