libgtp: Bump major LIBVERSION for recent API/ABI breakage

in Change-Id I68ae49a765828fa681054c68bf7f5e74dbe48ad2 we introduced
a new struct member that breks ABI and API.

Change-Id: Ie631880155513b1b78d1e9dd473dc5dc50e05943
diff --git a/gtp/Makefile.am b/gtp/Makefile.am
index 32025d6..7a0ff0a 100644
--- a/gtp/Makefile.am
+++ b/gtp/Makefile.am
@@ -1,7 +1,7 @@
 # This is _NOT_ the library release version, it's an API version.
 # Please read chapter "Library interface versions" of the libtool documentation
 # before making any modifications: https://www.gnu.org/software/libtool/manual/html_node/Versioning.html
-LIBVERSION=1:0:0
+LIBVERSION=2:0:0
 lib_LTLIBRARIES = libgtp.la
 
 include_HEADERS = gtp.h pdp.h gtpie.h