meson: do not use python.full_path() unnecessarily

The "python" variable is an external program and can be passed
directly to custom_target.  This avoids the need to look it up
multiple times, which was previously silent but is now explicit
in recent Meson versions.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

ui/meson.build

@@ -134,7 +134,7 @@ if have_system or xkbcommon.found()
                   output: output,
                   capture: true,
                   input: files('keycodemapdb/data/keymaps.csv'),
-                  command: [python.full_path(), files('keycodemapdb/tools/keymap-gen'),
+                  command: [python, files('keycodemapdb/tools/keymap-gen'),
                             'code-map',
                             '--lang', 'glib2',
                             '--varname', 'qemu_input_map_@0@_to_@1@'.format(e[0], e[1]),