[GTK] AcceleratedBackingStoreDMABuf compile error with GBM OFF

https://bugs.webkit.org/show_bug.cgi?id=255027

Reviewed by Michael Catanzaro.

Based on a patch originally by Jim Mason.

* Source/WebKit/PlatformGTK.cmake: Generate IPC messaging code for
  AcceleratedBackingStoreDMABuf only when USE_LIBGBM is defined.

Canonical link: https://commits.webkit.org/262787@main

Source/WebKit/PlatformGTK.cmake

@@ -52,13 +52,16 @@ list(APPEND WebKit_UNIFIED_SOURCE_LIST_FILES
 
 list(APPEND WebKit_MESSAGES_IN_FILES
     UIProcess/ViewGestureController
-
-    UIProcess/gtk/AcceleratedBackingStoreDMABuf
-
     WebProcess/gtk/GtkSettingsManagerProxy
     WebProcess/WebPage/ViewGestureGeometryCollector
 )
 
+if (USE_LIBGBM)
+    list(APPEND WebKit_MESSAGES_IN_FILES
+        UIProcess/gtk/AcceleratedBackingStoreDMABuf
+    )
+endif ()
+
 list(APPEND WebKit_DERIVED_SOURCES
     ${WebKitGTK_DERIVED_SOURCES_DIR}/InspectorGResourceBundle.c
     ${WebKitGTK_DERIVED_SOURCES_DIR}/WebKitDirectoryInputStreamData.cpp