LibC: Define the POSIX TZNAME_MAX limit

POSIX defines this as the "Maximum number of bytes supported for the
name of a timezone (not of the TZ variable)." It must have a minimum
value of _POSIX_TZNAME_MAX (6). The longest time zone name in the TZDB
is about 40 chars, so 64 is chosen here for a little wiggle room, and
to round up to a power of 2.

Userland/Libraries/LibC/limits.h

@@ -82,6 +82,8 @@
 
 #define LINK_MAX 4096
 
+#define TZNAME_MAX 64
+
 #ifdef __USE_POSIX
 #    include <bits/posix1_lim.h>
 #endif