Declare aom_write_one_yuv_frame() as static

It is only used in av1/encoder/encoder.c.

Change-Id: I0bf6500c8e14acd2b97f64cc4cc3e9041f479ef2

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index b4d4277..d396048 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -1930,7 +1930,7 @@
 }
 
 #ifdef OUTPUT_YUV_REC
-void aom_write_one_yuv_frame(AV1_COMMON *cm, YV12_BUFFER_CONFIG *s) {
+static void aom_write_one_yuv_frame(AV1_COMMON *cm, YV12_BUFFER_CONFIG *s) {
   uint8_t *src = s->y_buffer;
   int h = cm->height;
   if (yuv_rec_file == NULL) return;