macipgw: Incorrect sys header includes

[rdmark]

According to Alpine:

[36/364] Compiling C object contrib/macipgw/macipgw.p/tunnel_bsd.c.o
In file included from ../contrib/macipgw/tunnel_linux.c:26:
/usr/include/sys/errno.h:1:2: warning: #warning redirecting incorrect #include <sys/errno.h> to <errno.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/errno.h> to <errno.h>
      |  ^~~~~~~
[37/364] Compiling C object contrib/macipgw/macipgw.p/nbp_lkup_async.c.o
[38/364] Compiling C object contrib/macipgw/macipgw.p/main.c.o
Note: namesp. add : added namespace before processing              aecho
[39/364] Compiling C object contrib/macipgw/macipgw.p/macip.c.o
Note: namesp. add : added namespace before processing              afppasswd
[40/364] Compiling C object contrib/macipgw/macipgw.p/atp_input.c.o
[41/364] Compiling C object contrib/timelord/timelord.p/timelord.c.o
In file included from ../contrib/macipgw/nbp_lkup_async.c:38:
/usr/include/sys/signal.h:1:2: warning: #warning redirecting incorrect #include <sys/signal.h> to <signal.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/signal.h> to <signal.h>
      |  ^~~~~~~
[42/364] Compiling C object contrib/a2boot/a2boot.p/a2boot.c.o
In file included from ../contrib/macipgw/main.c:25:
/usr/include/sys/errno.h:1:2: warning: #warning redirecting incorrect #include <sys/errno.h> to <errno.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/errno.h> to <errno.h>
      |  ^~~~~~~
[43/364] Compiling C object etc/papd/papd.p/uam.c.o
In file included from ../contrib/macipgw/macip.c:26:
/usr/include/sys/errno.h:1:2: warning: #warning redirecting incorrect #include <sys/errno.h> to <errno.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/errno.h> to <errno.h>
      |  ^~~~~~~