Include-what-you-use ssl/internal.h.

The rest of ssl/ still includes things everywhere, but this at least fixes the
includes that were implicit from ssl/internal.h.

Change-Id: I7ed22590aca0fe78af84fd99a3e557f4b05f6782
Reviewed-on: https://boringssl-review.googlesource.com/4281
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/s3_both.c b/ssl/s3_both.c
index 20c1084..9ab8e1b 100644
--- a/ssl/s3_both.c
+++ b/ssl/s3_both.c
@@ -116,6 +116,7 @@
 #include <string.h>
 
 #include <openssl/buf.h>
+#include <openssl/err.h>
 #include <openssl/evp.h>
 #include <openssl/mem.h>
 #include <openssl/md5.h>