meson: Correct the option for disabled int10 from 'disabled' to 'false'

Fix meson_option.txt to align with the check of the int10 option against
'disabled', not 'false' in hw/xfree/meson.build, to see if it shouldn't be
built at all.

This keeps everything consistent that 'false' always turns things off.

Not noticed before as options weren't validated against choices until meson
0.43

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

hw/xfree86/meson.build

@@ -110,7 +110,7 @@ subdir('fbdevhw')
 if gbm_dep.found()
     subdir('glamor_egl')
 endif
-if int10 != 'disabled'
+if int10 != 'false'
     if int10 == 'x86emu'
         subdir('x86emu')
     endif