motion_est: Fix warning from zero_cmp() assignment

warning: incompatible
      pointer types assigning to 'me_cmp_func' (aka 'int (*)(struct
      MpegEncContext *, uint8_t *, uint8_t *, ptrdiff_t, int)') from 'int
      (MpegEncContext *, uint8_t *, uint8_t *, int, int)'
      [-Wincompatible-pointer-types]

libavcodec/motion_est.c

@@ -291,7 +291,7 @@ static int cmp_qpel(MpegEncContext *s, const int x, const int y, const int subx,
 #include "motion_est_template.c"
 
 static int zero_cmp(MpegEncContext *s, uint8_t *a, uint8_t *b,
-                    int stride, int h)
+                    ptrdiff_t stride, int h)
 {
     return 0;
 }