xcmd: include stdlib.h instead of sys/wait.h for WEXITSTATUS (#143)

sys/wait.h is not available on Windows.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Merge pull request #148 from masatake/use-WEXITSTATUS-in-stdlib-h

xcmd: include stdlib.h instead of sys/wait.h for WEXITSTATUS (#143)