opus.m4: fix -Wstrict-prototypes

Signed-off-by: Sam James <[email protected]>
Signed-off-by: Mark Harris <[email protected]>

Author: thesamesam

opus.m4

@@ -63,7 +63,7 @@ dnl
 #include <string.h>
 #include <opus.h>
 
-int main ()
+int main (void)
 {
   system("touch conf.opustest");
   return 0;