md4: build correctly with openssl without MD4

Reported-by: elsamuko at github
Fixes #3921
Closes #3922

lib/md4.c

@@ -38,6 +38,10 @@
 
 #include "curl_setup.h"
 
+#ifdef USE_OPENSSL
+#include <openssl/opensslconf.h>
+#endif
+
 /* The NSS, OS/400, and when not included, OpenSSL and mbed TLS crypto
  * libraries do not provide the MD4 hash algorithm, so we use this
  * implementation of it */