build: Add a proper pkgconfig libgtp.pc for the GTP library

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/configure.in b/configure.in
index 36a5cbe..3aa6909 100644
--- a/configure.in
+++ b/configure.in
@@ -115,5 +115,6 @@
                  sgsnemu/Makefile
                  src/Makefile
                  tests/Makefile
+                 libgtp.pc
                  openggsn.spec])
 AC_OUTPUT