build: correctly #define EXPERIMENTAL to 1
build: correctly #define EXPERIMENTAL to 1

Defining it to nothing gives problems in some scenarios if only #if is
used instead of #ifdef.

Signed-off-by: Felipe Contreras <de77a8b9cd239b204c19c52ed62e48ab47243362@nokia.com>

matsu/gst-openmax,freedesktop-unofficial-mirror/gstreamer__attic__gst-openmax,freedesktop-unofficial-mirror/gstreamer__attic__gst-openmax,matsu/gst-openmax