build: fix libavfilter check to include function prototype

Apparently the check error'ed on some compilers for missing prototype and
simply adding one fixes it.

waftools/fragments/libavfilter.c

@@ -1,4 +1,5 @@
 #include <libavfilter/avfilter.h>
+void vf_next_query_format(void);
 void vf_next_query_format() {}
 int main(void) {
     avfilter_register_all();