Fix musl build

Downloaded from
http://git.alpinelinux.org/cgit/aports/tree/main/xl2tpd/xl2tpd-compile.patch

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -u --recursive src.orig/xl2tpd-1.3.6/xl2tpd.c src/xl2tpd-1.3.6/xl2tpd.c
--- xl2tpd-1.3.6.orig/xl2tpd.c	2014-01-15 21:58:37.000000000 -0100
+++ xl2tpd-1.3.6/xl2tpd.c	2014-06-21 07:22:21.195278618 -0200
@@ -33,8 +33,6 @@
 #if (__GLIBC__ < 2)
 # if defined(FREEBSD) || defined(OPENBSD)
 #  include <sys/signal.h>
-# elif defined(LINUX)
-#  include <bsd/signal.h>
 # elif defined(SOLARIS)
 #  include <signal.h>
 # endif
