libav: Add libsdl to DEPENDS only when x11 is enabled

libav: Add libsdl to DEPENDS only when x11 is enabled

When we have opengl in distro features but not x11 and try to
build libav then it calls out to build libsdl which inturn has depependency on libglu
and libglu fails to build

ld: error: cannot find -lGL
| collect2: error: ld returned 1 exit status
| make: *** [libGLU.la] Error 1
| ERROR: oe_runmake failed

since libglu wants glx enabled in mesa and glx in mesa is (righly) enabled only when x11
is in distro features.

This breaks the dependency chain leading to this problem

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>