fsck.c: undefine temporary STR macro after use

In f417eed8cd (fsck: provide a function to parse fsck message IDs,
2015-06-22) the "STR" macro was introduced, but that short macro name
was not undefined after use as was done earlier in the same series for
the MSG_ID macro in c99ba492f1 (fsck: introduce identifiers for fsck
messages, 2015-06-22).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -100,6 +100,7 @@ static struct {
 	{ NULL, NULL, NULL, -1 }
 };
 #undef MSG_ID
+#undef STR
 
 static void prepare_msg_ids(void)
 {