x86/vp9dsp: add missing header include

Fixes make checkheaders

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/x86/vp9dsp_init.h

@@ -23,6 +23,8 @@
 #ifndef AVCODEC_X86_VP9DSP_INIT_H
 #define AVCODEC_X86_VP9DSP_INIT_H
 
+#include <libavcodec/vp9dsp.h>
+
 #define decl_fpel_func(avg, sz, bpp, opt) \
 void ff_vp9_##avg##sz##bpp##_##opt(uint8_t *dst, ptrdiff_t dst_stride, \
                                    const uint8_t *src, ptrdiff_t src_stride, \