TESTS: Fix build for filltab25.c

Give a return type to main(), so that filltab25.c compiles with
modern compilers.

tests/exp/filltab25.c

@@ -182,7 +182,7 @@ void switch_trees() {
 	p = sw;
 }
 
-main(int argc, char **argv) {
+int main(int argc, char **argv) {
 	int conns;
 	int i;