Merge pull request #20640 from BratishkaErik/std.c/setlocale-fix

std.c: setlocale fixes

lib/std/c.zig

@@ -1912,9 +1912,10 @@ 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;
+pub extern "c" fn setlocale(category: LC, locale: ?[*:0]const u8) ?[*:0]const u8;
 
 pub const getcontext = if (builtin.target.isAndroid())
     @compileError("android bionic libc does not implement getcontext")