waifulib: fwgslib: fix original list being changed

scripts/waifulib/fwgslib.py

@@ -51,11 +51,7 @@ def get_flags_by_type(flags, type, compiler):
 def filter_flags(conf, flags, required_flags, checkfunc, checkarg, compiler):
 	conf.msg('Detecting supported flags for %s' % (compiler), '...')
 
-	check_flags = required_flags
-
-	if compiler != 'msvc':
-		check_flags += ['-Werror']
-
+	check_flags = required_flags + (['-Werror'] if compiler != 'msvc' else [])
 	supported_flags = []
 
 	for f in flags: