avfilter/libmpcodecs/vf_eq: remove useless storage class specifier in empty declaration

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/libmpcodecs/vf_eq.c

@@ -31,7 +31,7 @@
 
 #include "libvo/video_out.h"
 
-static struct vf_priv_s {
+struct vf_priv_s {
         unsigned char *buf;
         int brightness;
         int contrast;