std.c.LC: mark enum as non-exhaustive

They are implementation-defined and can have values other than
hard-coded here. Also, standard permits other values not mentioned
there:

> Additional macro definitions, beginning with the characters LC_
> and an uppercase letter, may also be specified by the implementation.

Signed-off-by: Eric Joldasov <bratishkaerik@landless-city.net>

lib/std/c.zig

@@ -1912,6 +1912,7 @@ pub const LC = enum(c_int) {
     TELEPHONE = 10,
     MEASUREMENT = 11,
     IDENTIFICATION = 12,
+    _,
 };
 
 pub extern "c" fn setlocale(category: LC, locale: ?[*:0]const u8) ?[*:0]const u8;