Move kernel GTP support from ggsn/ to lib/

This way, the IP address / route handling between TUN devices and kernel
GTP can be shared, which will provide not only a unified codebase but
also a more consistent behavior.

This also paves the road for to use kernel GTP from sgsnemu in the future.

Related: OS#3214
Change-Id: Ic53a971136edd0d8871fbd6746d7b0090ce3a188
diff --git a/ggsn/Makefile.am b/ggsn/Makefile.am
index 6b2232f..eca385f 100644
--- a/ggsn/Makefile.am
+++ b/ggsn/Makefile.am
@@ -12,8 +12,4 @@
 endif
 
 osmo_ggsn_DEPENDENCIES = ../gtp/libgtp.la ../lib/libmisc.a
-osmo_ggsn_SOURCES = ggsn_vty.c ggsn.c ggsn.h gtp-kernel.h icmpv6.c icmpv6.h checksum.c checksum.h
-
-if ENABLE_GTP_KERNEL
-osmo_ggsn_SOURCES += gtp-kernel.c
-endif
+osmo_ggsn_SOURCES = ggsn_vty.c ggsn.c ggsn.h icmpv6.c icmpv6.h checksum.c checksum.h