drop xfixes build dependency

Its usage in the code was removed a looong time ago, back in commit
bccb03b.

Clean up the final reference, making `meson setup` a bit faster and
avoiding the need to install one more -dev package on some distros.

Author: eli-schwartz

meson.build

@@ -63,7 +63,6 @@ upower_glib = dependency('upower-glib', version: '>= 0.99.11')
 wacom = dependency('libwacom', version: '>= 0.7', required: get_option('use_wacom'))
 x11 = dependency('x11')
 xext = dependency('xext')
-xfixes = dependency('xfixes')
 xi = dependency('xi')
 
 # currently only used for the wacom plugin