resize.c: make resize_horz_dir static

fixes -Wmissing-protoypes warning

Bug: aomedia:3416
Change-Id: I46a1ac07200fb1df214ab1ed71d0e4fcbf267e8e
diff --git a/av1/common/resize.c b/av1/common/resize.c
index 2b48b9f..727f84f 100644
--- a/av1/common/resize.c
+++ b/av1/common/resize.c
@@ -543,8 +543,9 @@
   return mem_status;
 }
 
-void resize_horz_dir(const uint8_t *const input, int in_stride, uint8_t *intbuf,
-                     int height, int filtered_length, int width2) {
+static INLINE void resize_horz_dir(const uint8_t *const input, int in_stride,
+                                   uint8_t *intbuf, int height,
+                                   int filtered_length, int width2) {
   for (int i = 0; i < height; ++i)
     down2_symeven(input + in_stride * i, filtered_length, intbuf + width2 * i);
 }