meson: Sync the name of INPUTTHREAD conf macro with autotools build

The rest of dix code uses INPUTTHREAD, so having a different name in
meson configuration was an oversight.

Fixes: https://gitlab.freedesktop.org/xorg/xserver/-/issues/936

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>

include/meson.build

@@ -60,7 +60,7 @@ else
       enable_input_thread = false
   endif
 endif
-conf_data.set('HAVE_INPUTTHREAD', enable_input_thread)
+conf_data.set('INPUTTHREAD', enable_input_thread ? '1' : false)
 
 if cc.compiles('''
     #define _GNU_SOURCE 1