clean up implicit declaration warnings for neon

Change-Id: I6ca2d89f355839c4c770773c09fc69dcea7c1406
warning: implicit declaration of function
  'vp8_variance_halfpixvar16x16_[h|v|hv]_neon'
  'vp8_sub_pixel_variance16x16_neon_func'

vp8/encoder/arm/variance_arm.c

@@ -9,6 +9,7 @@
  */
 
 #include "vpx_config.h"
+#include "variance.h"
 
 #if HAVE_ARMV7