sandbox: only test for sdl libs when sdl is enabled
Otherwise we get the following warning when sdl is not installed
on the compile host:
Package sdl was not found in the pkg-config search path.
Perhaps you should add the directory containing `sdl.pc'
to the PKG_CONFIG_PATH environment variable
No package 'sdl' found
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>