mediastreamer: fix build with libX11 and sdl without x11 support

Fixes http://autobuild.buildroot.net/results/a8f2db4a40c0f53c0d25b2632fe87c2ce136f770



Mediastreamer assumes SDL has X11 support if SDL and libX11 are found,
which is not necessarily true in BR.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>