t/: Use "int main(void)" for the main C signature

This commit updates a few tests where we aren't using the correct C
signature for main() or are not using any of its parameters (in the
case of t/version/version.c.)

t/generated-code2/test-generated-code2.c

@@ -2370,7 +2370,7 @@ static Test tests[] =
 };
 #define n_tests (sizeof(tests)/sizeof(Test))
 
-int main ()
+int main(void)
 {
   unsigned i;
   for (i = 0; i < n_tests; i++)

t/version/version.c

@@ -35,7 +35,7 @@
 #include "protobuf-c.h"
 
 int
-main(int argc, char **argv)
+main(void)
 {
 	printf("PACKAGE_VERSION = %s\n",
 	       PACKAGE_VERSION);