meson: Add the x(org)proto headers to the include path

Without this meson won't properly use headers installed into a
non-default location.

Signed-off-by: Adam Jackson <ajax@redhat.com>

meson.build

@@ -455,6 +455,7 @@ inc = include_directories(
     'randr',
     'render',
     'xfixes',
+    xproto_dep.get_pkgconfig_variable('includedir')
 )
 
 glx_inc = include_directories('glx')