pull in openssl/conf.h for OPENSSL_config() prototype

--- openbsd-compat/openssl-compat.c
+++ openbsd-compat/openssl-compat.c
@@ -59,6 +59,7 @@
 #endif
 
 #ifdef	USE_OPENSSL_ENGINE
+#include <openssl/conf.h>
 void
 ssh_SSLeay_add_all_algorithms(void)
 {