std.valgrind.callgrind: fix string type

Fixes the error when using std.vallgrind.callgrind:
error: expected type '[2]u8', found '*const [2:0]u8'

lib/std/valgrind/callgrind.zig

@@ -2,7 +2,7 @@ const std = @import("../std.zig");
 const valgrind = std.valgrind;
 
 pub const CallgrindClientRequest = enum(usize) {
-    DumpStats = valgrind.ToolBase("CT"),
+    DumpStats = valgrind.ToolBase("CT".*),
     ZeroStats,
     ToggleCollect,
     DumpStatsAt,