Fix #129985: macOS headless build fails due to missing CGL context symbols

When making a `headless` macOS build, linking would fail complaining
about missing `GHOST_ContextCGL` symbols, which are required by the Metal
backend. This commit fixes this issue by compiling the macOS CGL GHOST
Context if the Metal backend is enabled, instead of only depending on
`WITH_HEADLESS`, similarly to what's done on Linux/other *nix systems.

Pull Request: https://projects.blender.org/blender/blender/pulls/130847

intern/ghost/CMakeLists.txt

@@ -185,12 +185,6 @@ elseif(APPLE AND NOT WITH_GHOST_X11)
     intern/GHOST_WindowViewCocoa.hh
   )
 
-  list(APPEND SRC
-    intern/GHOST_ContextCGL.mm
-
-    intern/GHOST_ContextCGL.hh
-  )
-
   if(WITH_INPUT_NDOF)
     list(APPEND SRC
       intern/GHOST_NDOFManagerCocoa.mm
@@ -531,6 +525,16 @@ if(UNIX AND NOT APPLE)
   endif()
 endif()
 
+if(APPLE)
+  if(WITH_METAL_BACKEND)
+    list(APPEND SRC
+      intern/GHOST_ContextCGL.mm
+
+      intern/GHOST_ContextCGL.hh
+    )
+  endif()
+endif()
+
 if(APPLE)
   list(APPEND SRC
     intern/GHOST_SystemPathsCocoa.hh