meson: set minimum Meson version to 0.56.0

Set the minimum required version of Meson based on the highest version
feature used, as detected by meson-setup.

* 0.56.0: {'meson.project_build_root'}

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

meson.build

@@ -2,6 +2,7 @@ project('dtc', 'c',
   version: '1.7.0',
   license: ['GPL2+', 'BSD-2'],
   default_options: 'werror=true',
+  meson_version: '>=0.56.0'
 )
 
 cc = meson.get_compiler('c')