Use 'private-code' for wayland-scanner code generation

They deprecated 'code' for 'public-code', but suggest using 'private-code'...

protocol/meson.build

@@ -8,10 +8,17 @@ wayland_scanner_server = generator(
 	arguments: ['server-header', '@INPUT@', '@OUTPUT@'],
 )
 
+# should check wayland_scanner's version, but it is hard to get
+if wayland_server.version().version_compare('>=1.14.91')
+	code_type = 'private-code'
+else
+	code_type = 'code'
+endif
+
 wayland_scanner_code = generator(
 	wayland_scanner,
 	output: '@BASENAME@-protocol.c',
-	arguments: ['code', '@INPUT@', '@OUTPUT@'],
+	arguments: [code_type, '@INPUT@', '@OUTPUT@'],
 )
 
 wayland_scanner_client = generator(