server: fix button signedness in wl_grab_interface

wl_input_device::grab_button is unsigned but the button parameter to
wl_grab_interface::button is signed. This lead to a warning in
data-device.c.

The button number is unsigned in the protocol, so make it unsigned in
the wl_grab_interface API, too. Fixes the compiler warning "comparison
between signed and unsigned integer expressions".

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>