tools_common.h: fix get_vpx_decoder_count() proto

silences a missing-prototype warning

Change-Id: I99f8b4a9b7df836208e93170159733c5b5c2d881
diff --git a/tools_common.h b/tools_common.h
index db6e268..a87e814 100644
--- a/tools_common.h
+++ b/tools_common.h
@@ -135,7 +135,7 @@
 const VpxInterface *get_vpx_encoder_by_index(int i);
 const VpxInterface *get_vpx_encoder_by_name(const char *name);
 
-int get_vpx_decoder_count();
+int get_vpx_decoder_count(void);
 const VpxInterface *get_vpx_decoder_by_index(int i);
 const VpxInterface *get_vpx_decoder_by_name(const char *name);
 const VpxInterface *get_vpx_decoder_by_fourcc(uint32_t fourcc);