matchbox-lib: Append -lX11 to the linker

Fixes building with binutils-2.22
http://autobuild.buildroot.net/results/b4b26dd9c7d7bebe45638ba125fb49a5f5d0cc14/build-end.log



Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>