meson: Fix xkbcommon-x11.pc Requires

The meson-generated pkgconfig file was missing Requires and
Requires.private.

[ran: adjust for older Meson versions.]

Signed-off-by: Jan Alexander Steffens (heftig) <jan.steffens@gmail.com>
Signed-off-by: Ran Benita <ran234@gmail.com>

meson.build

@@ -258,6 +258,8 @@ You can disable X11 support with -Denable-x11=false.''')
         libraries: libxkbcommon_x11,
         version: meson.project_version(),
         description: 'XKB API common to servers and clients - X11 support',
+        requires: 'xkbcommon',
+        requires_private: 'xcb>=1.10 xcb-xkb>=1.10',
     )
 endif