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/evp/example_sign.c b/crypto/evp/example_sign.c
index df7156b..847330d 100644
--- a/crypto/evp/example_sign.c
+++ b/crypto/evp/example_sign.c
@@ -95,7 +95,7 @@
 };
 
 
-int example_EVP_DigestSignInit() {
+int example_EVP_DigestSignInit(void) {
   int ret = 0;
   EVP_PKEY *pkey = NULL;
   RSA *rsa = NULL;
@@ -154,7 +154,7 @@
   return ret;
 }
 
-int example_EVP_DigestVerifyInit() {
+int example_EVP_DigestVerifyInit(void) {
   int ret = 0;
   EVP_PKEY *pkey = NULL;
   RSA *rsa = NULL;
@@ -193,7 +193,7 @@
   return ret;
 }
 
-int main() {
+int main(void) {
   if (!example_EVP_DigestSignInit()) {
     fprintf(stderr, "EVP_DigestSignInit failed\n");
     return 1;