meson.build: fix pkgconfig deprecation warning

meson.build: fix pkgconfig deprecation warning

../meson.build:239: DEPRECATION: Library xkbcommon was passed to the "libraries"
keyword argument of a previous call to generate() method instead of first
positional argument. Adding xkbcommon to "Requires" field, but this is a
deprecated behaviour that will change in a future version of Meson. Please
report the issue if this warning cannot be avoided in your case.

This was introduced in meson 0.49.0

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>