clang: fix libString

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>

sys/src/libString/libString.json

@@ -1,7 +1,6 @@
 {
 	"libString": {
 		"Cflags": [
-			"-fplan9-extensions",
 			"-Werror"
 		],
 		"Include": [