[CMAKE] Build xboxvmp in Xbox-builds only

This prevents the creation of a fake Xbox GPU device on non-Xbox builds.

CORE-17529

win32ss/drivers/miniport/CMakeLists.txt

@@ -2,13 +2,11 @@
 if(SARCH STREQUAL "pc98")
     # Actual binary filename is vga.sys
     add_subdirectory(pc98vid)
+elseif(SARCH STREQUAL "xbox")
+    add_subdirectory(xboxvmp)
 else()
     add_subdirectory(vbe)
     add_subdirectory(vga)
     add_subdirectory(vga_new)
     add_subdirectory(vmx_svga)
-
-    if(ARCH STREQUAL "i386")
-        add_subdirectory(xboxvmp)
-    endif()
 endif()