Move icmpv6 and checksum files from ggsn/ dir to lib/

They will be required by sgsnemu to implement ICMPv6 Router
Soliciations.

Change-Id: Ie878604f0fc0169cc98a1e9eee64b14d76be2c45
diff --git a/ggsn/Makefile.am b/ggsn/Makefile.am
index ca389f0..eea7c6e 100644
--- a/ggsn/Makefile.am
+++ b/ggsn/Makefile.am
@@ -12,4 +12,4 @@
 endif
 
 osmo_ggsn_DEPENDENCIES = ../gtp/libgtp.la ../lib/libmisc.a
-osmo_ggsn_SOURCES = ggsn_main.c ggsn_vty.c ggsn.c ggsn.h sgsn.c sgsn.h icmpv6.c icmpv6.h checksum.c checksum.h pco.c pco.h
+osmo_ggsn_SOURCES = ggsn_main.c ggsn_vty.c ggsn.c ggsn.h sgsn.c sgsn.h pco.c pco.h