hwcontext_vulkan: fix make checkheaders fail

hwcontext_vulkan: fix make checkheaders fail

make checkheaders will get error as follow:
CC	libavutil/hwcontext_vulkan.h.o
In file included from libavutil/hwcontext_vulkan.h.c:1:
./libavutil/hwcontext_vulkan.h:130:23: error: ‘AV_NUM_DATA_POINTERS’ undeclared here (not in a function)
  130 |     void *alloc_pnext[AV_NUM_DATA_POINTERS];
      |                       ^~~~~~~~~~~~~~~~~~~~
./libavutil/hwcontext_vulkan.h:199:43: warning: ‘enum AVPixelFormat’ declared inside parameter list will not be visible outside of this definition or declaration

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>