strptime: add missing const for "GMT" and "UTC".

Change-Id: I46d7dabddc87d40ecc2ddb9290d4e81db0b5822d

Author: enh-google

libc/tzcode/strptime.c

@@ -68,8 +68,8 @@
 #define FIELD_TM_YDAY	(1 << 3)
 #define FIELD_TM_YEAR	(1 << 4)
 
-static char gmt[] = { "GMT" };
-static char utc[] = { "UTC" };
+static const char gmt[] = { "GMT" };
+static const char utc[] = { "UTC" };
 /* RFC-822/RFC-2822 */
 static const char * const nast[5] = {
        "EST",    "CST",    "MST",    "PST",    "\0\0\0"