nvbaseenc: fix build warnings with HAVE_NVENC_GST_GL

fix following error:
gstnvbaseenc.c: In function ‘gst_nv_base_enc_set_context’:
gstnvbaseenc.c:451:17: error: unused variable ‘nvenc’ [-Werror=unused-variable]
   GstNvBaseEnc *nvenc = GST_NV_BASE_ENC (element);
                 ^

https://bugzilla.gnome.org/show_bug.cgi?id=794896

sys/nvenc/gstnvbaseenc.c

@@ -448,9 +448,9 @@ gst_nv_base_enc_open (GstVideoEncoder * enc)
 static void
 gst_nv_base_enc_set_context (GstElement * element, GstContext * context)
 {
+#if HAVE_NVENC_GST_GL
   GstNvBaseEnc *nvenc = GST_NV_BASE_ENC (element);
 
-#if HAVE_NVENC_GST_GL
   gst_gl_handle_set_context (element, context,
       (GstGLDisplay **) & nvenc->display,
       (GstGLContext **) & nvenc->other_context);
@@ -465,7 +465,9 @@ gst_nv_base_enc_set_context (GstElement * element, GstContext * context)
 static gboolean
 gst_nv_base_enc_sink_query (GstVideoEncoder * enc, GstQuery * query)
 {
+#if HAVE_NVENC_GST_GL
   GstNvBaseEnc *nvenc = GST_NV_BASE_ENC (enc);
+#endif
 
   switch (GST_QUERY_TYPE (query)) {
 #if HAVE_NVENC_GST_GL