libweston: use new versioning scheme

Use the documented libweston-$major.so.0.$minor.$patch scheme.

An (almost) identical one is used by GLIB, GDK{2,3}, QT5, json-glib and
others.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
diff --git a/compositor/weston.pc.in b/compositor/weston.pc.in
index 09e8580..6890a77 100644
--- a/compositor/weston.pc.in
+++ b/compositor/weston.pc.in
@@ -8,5 +8,5 @@
 Name: Weston Plugin API
 Description: Header files for Weston plugin development
 Version: @WESTON_VERSION@
-Requires.private: libweston-@LIBWESTON_ABI_VERSION@
+Requires.private: libweston-@LIBWESTON_MAJOR@
 Cflags: -I${includedir}/weston