configure.ac: Require libosmo-netif 0.6.0

Current code uses APIs like osmux_xfrm_output_init2() and osmo_amr_*(),
that are only available in libosmo-netif 0.6.0 onwards. Let's update
configure.ac accordingly.

Change-Id: I3bc0196bb6b5c5e5cf96935422fd56c4582ed7f5
diff --git a/configure.ac b/configure.ac
index b4858c4..124f1e6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -42,7 +42,7 @@
 PKG_CHECK_MODULES(LIBOSMOCORE, libosmocore >= 1.0.0)
 PKG_CHECK_MODULES(LIBOSMOGSM, libosmogsm >= 1.0.0)
 PKG_CHECK_MODULES(LIBOSMOVTY, libosmovty >= 1.0.0)
-PKG_CHECK_MODULES(LIBOSMONETIF, libosmo-netif >= 0.4.0)
+PKG_CHECK_MODULES(LIBOSMONETIF, libosmo-netif >= 0.6.0)
 
 AC_ARG_ENABLE(sanitize,
 	[AS_HELP_STRING(