SSL: fixed typo introduced in 8e335c2ac447.
diff --git a/external/njs_openssl.h b/external/njs_openssl.h
index dd2f34c..002c6a2 100644
--- a/external/njs_openssl.h
+++ b/external/njs_openssl.h
@@ -21,7 +21,7 @@
 #include <openssl/rand.h>
 #include <openssl/crypto.h>
 
-#if EVP_PKEY_HKDF
+#ifdef EVP_PKEY_HKDF
 #include <openssl/kdf.h>
 #endif