lxd/include: include sys/wait.h in macro.h
lxd/include: include sys/wait.h in macro.h

P_PIDFD is defined in sys/wait.h, but isn't included anywhere explicitly as
far as I can tell, so we'll always get this define even if the kernel
headers have it.

This fixes an inscrutable compiler error on Alpine for me:

# github.com/lxc/lxd/shared/idmap
In file included from ./../../lxd/include/memory_utils.h:11,
                 from /root/go/pkg/mod/github.com/lxc/lxd@v0.0.0-20210607165116-5a9c2f17d640/shared/idmap/shift_linux.go:41:
./../../lxd/include/macro.h:279:17: error: expected identifier before numeric constant
  279 | #define P_PIDFD 3
      |                 ^

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.pizza>

stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,atxwebs/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd