packagegroup-fsl: Fix build without gst-fsl-plugin support

There are many situations we want to avoid the inclusion of
gst-fsl-plugin into a build (e.g using Linux kernel mainline).

The the package group need to dynamically adapt to it and avoid
adding the gplay when gst-fsl-plugin is not included or we'll get
build failures due missing kernel headers.

We fix it checking if gst-fsl-plugin is included in
MACHINE_GSTREAMER_PLUGIN variable and adding gplay only when it is
found.

Change-Id: Ia4240c44f481761d00f7ad7c8162e392e1001c50
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>