tools_common.h: fix get_vpx_decoder_count() proto

silences a missing-prototype warning

Change-Id: I99f8b4a9b7df836208e93170159733c5b5c2d881
diff --git a/tools_common.c b/tools_common.c
index 7e22217..901734e 100644
--- a/tools_common.c
+++ b/tools_common.c
@@ -170,7 +170,7 @@
 #endif
 };
 
-int get_vpx_decoder_count() {
+int get_vpx_decoder_count(void) {
   return sizeof(vpx_decoders) / sizeof(vpx_decoders[0]);
 }