Put WL_DEPRECATED in front of the function declarations

This fixes the following clang error when using C23:

../src/wayland-server-core.h:680:41: error: 'deprecated' attribute cannot be applied to types
  680 |                      int32_t stride, uint32_t format) WL_DEPRECATED;
      |                                                       ^
../src/wayland-util.h:52:25: note: expanded from macro 'WL_DEPRECATED'
   52 | #define WL_DEPRECATED [[deprecated]]
      |                         ^

Signed-off-by: Kirill Primak <vyivel@eclair.cafe>