Make choose_frame_source() static

Change-Id: Ic53649c92985fa4b1d6b7edd2dae98214c9481a8

av1/encoder/encode_strategy.c

diff --git a/av1/encoder/encode_strategy.c b/av1/encoder/encode_strategy.c
index 20687f3..8c71906 100644
--- a/av1/encoder/encode_strategy.c
+++ b/av1/encoder/encode_strategy.c
@@ -551,7 +551,7 @@
 // Do some setup associated with the chosen source
 // temporal_filtered, flush, and frame_update_type are outputs.
 // Return the frame source, or NULL if we couldn't find one
-struct lookahead_entry *choose_frame_source(
+static struct lookahead_entry *choose_frame_source(
     AV1_COMP *const cpi, int *const temporal_filtered, int *const flush,
     struct lookahead_entry **last_source, FRAME_UPDATE_TYPE *frame_update_type,
     EncodeFrameParams *const frame_params) {