fix type-mismatched declarations of __nl_langinfo_l in source files

obviously the type "should be" const, but it inherited non-const from
the standard nl_langinfo_l.

src/time/asctime_r.c

@@ -5,7 +5,7 @@
 #include "atomic.h"
 #include "libc.h"
 
-const char *__nl_langinfo_l(nl_item, locale_t);
+char *__nl_langinfo_l(nl_item, locale_t);
 
 char *__asctime_r(const struct tm *restrict tm, char *restrict buf)
 {

src/time/strftime.c

@@ -9,7 +9,7 @@
 #include "libc.h"
 #include "time_impl.h"
 
-const char *__nl_langinfo_l(nl_item, locale_t);
+char *__nl_langinfo_l(nl_item, locale_t);
 
 static int is_leap(int y)
 {