Fix build with opaque EVP structures

Index: lib/crypto/c_src/crypto.c
--- lib/crypto/c_src/crypto.c.orig
+++ lib/crypto/c_src/crypto.c
@@ -93,6 +93,7 @@
 #ifdef LIBRESSL_VERSION_NUMBER
 /* A macro to test on in this file */
 #define HAS_LIBRESSL
+#define HAS_EVP_PKEY_CTX
 #endif
 
 #ifdef HAS_LIBRESSL
