meson.build: Add support for local non-root install
If configured with
meson --prefix ~/foo then it's likely the user will
ninja install as non-root. This is ideal for development
but will fail if
dependency('gnome-keybindings', required: false) is
found, because that's almost certainly in
/usr/share where the user
can't install to.
So make sure any custom prefix applies to the keybindings directory too,
ninja install will succeed without errors.