weston.pc: Properly specify Requires.private

We include wayland-server.h, pixman.h and xkbcommon.h in compositor.h,
but they are not required for linking if the plugin doesn’t use them.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>

src/weston.pc.in

@@ -8,4 +8,5 @@ pkglibexecdir=${libexecdir}/@PACKAGE@
 Name: Weston Plugin API
 Description: Header files for Weston plugin development
 Version: @WESTON_VERSION@
+Requires.private: wayland-server pixman-1 xkbcommon
 Cflags: -I${includedir}