Fix compiling warnings in example code

The example code in EVP_DigestInit.pod generates warnings if users try
to compile it.

[skip ci]

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7362)

1 files changed, 3 insertions, 2 deletions

diff --git a/doc/man3/EVP_DigestInit.pod b/doc/man3/EVP_DigestInit.pod
index 0fedd17ce6..5ecbcc5e89 100644
--- a/doc/man3/EVP_DigestInit.pod
+++ b/doc/man3/EVP_DigestInit.pod
@@ -310,16 +310,17 @@ This example digests the data "Test Message\n" and "Hello World\n", using the
 digest name passed on the command line.
 
  #include <stdio.h>
+ #include <string.h>
  #include <openssl/evp.h>
 
- main(int argc, char *argv[])
+ int main(int argc, char *argv[])
  {
      EVP_MD_CTX *mdctx;
      const EVP_MD *md;
      char mess1[] = "Test Message\n";
      char mess2[] = "Hello World\n";
      unsigned char md_value[EVP_MAX_MD_SIZE];
-     int md_len, i;
+     unsigned int md_len, i;
 
      if (argv[1] == NULL) {
          printf("Usage: mdtest digestname\n");