Removing OpenSSL dependency for plugins.

Compiling a plugin required having access to OpenSSL headers because
they were pulled in by Bro headers that the plugin had to include.
Removinng then OpenSSL dependency from those Bro headers.

I'm also reverting a4e5591e. This is a different fix for the same
problem, and reverting that commit gives us a test case. :-)

src/main.cc

@@ -18,7 +18,8 @@ extern "C" {
 }
 #endif
 
-#include <openssl/md5.h>
+#include <openssl/ssl.h>
+#include <openssl/err.h>
 
 extern "C" void OPENSSL_add_all_algorithms_conf(void);