libxtables: add -I/-L flags to pkgconfig files

These are needed in case iptables gets installed into a non-standard
path. It also enables automatic detection of these locations from 3rd
party programs via pkgconfig.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/xtables.pc.in b/xtables.pc.in
index 7246d7b..fa6f33b 100644
--- a/xtables.pc.in
+++ b/xtables.pc.in
@@ -8,4 +8,5 @@
 Name:		xtables
 Description:	Shared Xtables code for extensions and iproute2
 Version:	@PACKAGE_VERSION@
-Libs:		-lxtables
+Cflags:		-I${includedir}
+Libs:		-L${libdir} -lxtables