In test cases/frameworks/8 flex, ensure yyparse() is prototyped

Old versions of bison (including the one shipped with macOS) do not
prototype yyparse(), just in case you are using a K&R compiler...

test cases/frameworks/8 flex/prog.c

@@ -6,6 +6,8 @@
 #include<stdio.h>
 #include<stdlib.h>
 
+extern int yyparse();
+
 int main(int argc, char **argv) {
     /*
     int input;