fix building with libressl

patch from Gentoo Bug 614460, credit to hexumg@gmail.com.
diff --git a/libarchive/archive_openssl_hmac_private.h b/libarchive/archive_openssl_hmac_private.h
index 2deeb5f..59f95b8 100644
--- a/libarchive/archive_openssl_hmac_private.h
+++ b/libarchive/archive_openssl_hmac_private.h
@@ -28,7 +28,7 @@
 #include <openssl/hmac.h>
 #include <openssl/opensslv.h>
 
-#if OPENSSL_VERSION_NUMBER < 0x10100000L
+#if OPENSSL_VERSION_NUMBER < 0x10100000L || defined(LIBRESSL_VERSION_NUMBER)
 #include <stdlib.h> /* malloc, free */
 #include <string.h> /* memset */
 static inline HMAC_CTX *HMAC_CTX_new(void)