xl2tpd: fix build with musl

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

net/xl2tpd/patches/120-no-bsd-signal-in-musl.patch

@@ -0,0 +1,13 @@
+Index: xl2tpd-1.3.6/xl2tpd.c
+===================================================================
+--- xl2tpd-1.3.6.orig/xl2tpd.c
++++ xl2tpd-1.3.6/xl2tpd.c
+@@ -30,7 +30,7 @@
+ #include <errno.h>
+ #include <unistd.h>
+ #include <time.h>
+-#if (__GLIBC__ < 2)
++#if defined (__GLIBC__) && (__GLIBC__ < 2)
+ # if defined(FREEBSD) || defined(OPENBSD)
+ #  include <sys/signal.h>
+ # elif defined(LINUX)