Add include directives to get WIFEXITED() and WEXITSTATUS() macros defined.

Fixes warnings:
  CC    migration-exec.o
/src/qemu/migration-exec.c: In function `exec_close':
/src/qemu/migration-exec.c:53: warning: implicit declaration of function `WIFEXITED'
/src/qemu/migration-exec.c:54: warning: implicit declaration of function `WEXITSTATUS'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>