plugin: fix macro processor check
plugin: fix macro processor check

Instead of #ifdef it should be used #if becasuse USE_GST_GL_HELPERS is always
defined in config.h, but it would be 0 or 1 depending on the configure output.

https://bugzilla.gnome.org/show_bug.cgi?id=765702

01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi