Explicitely add "opt_compat.h" to kern_exec.c: fix powerpc LINT builds.

sys/ptrace.h includes sys/signal.h, which includes sys/_sigset.h.
Note that sys/_sigset.h only defines osigset_t if COMPAT_43 was defined.

Two lines later, sys/ptrace.h includes machine/reg.h, which in case of
powerpc, includes opt_compat.h.

After the include headers reordering in r311345, we have sys/ptrace.h
included before sys/sysproto.h.

If COMPAT_43 was requested in the kernel config, the result is that
sys/_sigset.h does not define osigset_t, but sys/sysproto.h sees
COMPAT_43 and uses osigset_t.

Fix this by explicitely including opt_compat.h to cover the whole
kern/kern_exec.c scope.

Sponsored by: The FreeBSD Foundation