tools_common.h: fix get_vpx_encoder_count() proto

silences a missing-prototype warning

Change-Id: Icf5c7f1f3e8ae9792276068fb3c0fd04b40fc7ad
diff --git a/tools_common.h b/tools_common.h
index b92980a..db6e268 100644
--- a/tools_common.h
+++ b/tools_common.h
@@ -131,7 +131,7 @@
   vpx_codec_iface_t *(*const codec_interface)();
 } VpxInterface;
 
-int get_vpx_encoder_count();
+int get_vpx_encoder_count(void);
 const VpxInterface *get_vpx_encoder_by_index(int i);
 const VpxInterface *get_vpx_encoder_by_name(const char *name);