libsdl2: add missing dependency on libxkbcommon for PACKAGECONFIG[wayland]

libsdl2 depends on libxkbcommon when enable wayland support.

Can verify it with the following steps:
bitbake libxkbcommon -c clean
bitbake libsdl2 -c cleansstate && bitbake libsdl2

config.log:
[snip]
configure:19347: checking for Wayland support
configure:19357: result: no
[snip]

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>