third_party/fuchsia: Use fully qualified name

Change-Id: Ia592b2aec7bfb604139f1f10990f3f14994fc8af
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/105168
Commit-Queue: Vadim Spivak <vadims@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>

third_party/fuchsia/repo/sdk/lib/fit/include/lib/fit/function_internal.h

@@ -372,7 +372,7 @@ class function_base<inline_target_size, require_inline, Result(Args...)> {
  "Alignment of Callable must be <= alignment of max_align_t.");
  static_assert(!require_inline || sizeof(DecayedCallable) <= inline_target_size,
  "Callable too large to store inline as requested.");
- if (is_null(target)) {
+ if (::fit::is_null(target)) {
  initialize_null_target();
  } else {
  ops_ = &target_type<DecayedCallable>::ops;