SSL: add missing include.

ngx_event_openssl.c uses HMAC_Init_ex(), so it should include hmac.h.

Change-Id: I0d6516298741bf6c4e9466d596532a127ca9b8ee
Signed-off-by: Matthew Braithwaite <mab@google.com>
Reviewed-on: https://nginx-review.googlesource.com/3200
Reviewed-by: Piotr Sikora <piotrsikora@google.com>

src/event/ngx_event_openssl.h

diff --git a/src/event/ngx_event_openssl.h b/src/event/ngx_event_openssl.h
index 92bb34e..0c8a5c6 100644
--- a/src/event/ngx_event_openssl.h
+++ b/src/event/ngx_event_openssl.h
@@ -22,6 +22,7 @@
 #include <openssl/engine.h>
 #endif
 #include <openssl/evp.h>
+#include <openssl/hmac.h>
 #ifndef OPENSSL_NO_OCSP
 #include <openssl/ocsp.h>
 #endif