Convert all zero-argument functions to '(void)'

Otherwise, in C, it becomes a K&R function declaration which doesn't actually
type-check the number of arguments.

Change-Id: I0731a9fefca46fb1c266bfb1c33d464cf451a22e
Reviewed-on: https://boringssl-review.googlesource.com/1582
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/modes/gcm_test.c b/crypto/modes/gcm_test.c
index 8c52b85..a112431 100644
--- a/crypto/modes/gcm_test.c
+++ b/crypto/modes/gcm_test.c
@@ -413,7 +413,7 @@
   return ret;
 }
 
-int main() {
+int main(void) {
   int ret = 0;
   unsigned i;