[gardening] Fix fuchsia build breakage

Follow-up to https://dart.googlesource.com/sdk/+/0ea04d407d74503fb7a7ae581727523ec979a2ed

TEST=ci

Change-Id: I335b3a0bdeed6b0d1d58bf9ea2ec671e7c9cf7c3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/340381
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

runtime/vm/os_fuchsia.cc

@@ -318,7 +318,7 @@ bool InitializeTZData() {
   // wrong.
   //
   // Try the new dir first, sub with the old fallback.
-  char* tz_dirname = kICUTZDataDir2;
+  const char* tz_dirname = kICUTZDataDir2;
   int fd = openat(AT_FDCWD, tz_dirname, O_RDONLY);
   if (fd < 0) {
     tz_dirname = kICUTZDataDir;