move declaration and apply hidden visibility to __utc string

src/time/gmtime_r.c

@@ -2,8 +2,6 @@
 #include <errno.h>
 #include "libc.h"
 
-extern const char __utc[];
-
 struct tm *__gmtime_r(const time_t *restrict t, struct tm *restrict tm)
 {
 	if (__secs_to_tm(*t, tm) < 0) {

src/time/time_impl.h

@@ -8,3 +8,4 @@ hidden const char *__tm_to_tzname(const struct tm *);
 hidden int __secs_to_tm(long long, struct tm *);
 hidden void __secs_to_zone(long long, int, int *, long *, long *, const char **);
 hidden const char *__strftime_fmt_1(char (*)[100], size_t *, int, const struct tm *, locale_t, int);
+extern hidden const char __utc[];

src/time/timegm.c

@@ -2,8 +2,6 @@
 #include "time_impl.h"
 #include <errno.h>
 
-extern const char __utc[];
-
 time_t timegm(struct tm *tm)
 {
 	struct tm new;