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

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/Makefile.am b/Makefile.am
index 581e1bd..960ca07 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,2 +1,5 @@
 ## Process this file with automake to produce Makefile.in
 SUBDIRS = lib gtp ggsn sgsnemu doc
+
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = libgtp.pc