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: NJun Zhao <barryjzhao@tencent.com>

libavutil/hwcontext_vulkan.h

@@ -21,6 +21,9 @@
 
 #include <vulkan/vulkan.h>
 
+#include "pixfmt.h"
+#include "frame.h"
+
 /**
  * @file
  * API-specific header for AV_HWDEVICE_TYPE_VULKAN.