Depend on libgtp >= 0.92, as previous versions don't have gtpie.h installed

this fixes compilation for people having older gtp libraries installed.
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 098e5b4..df6ca67 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -75,7 +75,7 @@
 
 
 found_libgtp=yes
-PKG_CHECK_MODULES(LIBGTP, libgtp, , found_libgtp=no)
+PKG_CHECK_MODULES(LIBGTP, libgtp >= 0.92, , found_libgtp=no)
 AM_CONDITIONAL(HAVE_LIBGTP, test "$found_libgtp" = yes)
 AC_SUBST(found_libgtp)