Add libfuzzer_exports.h to Tint fuzzer common code

BUG=chromium:1353969

Change-Id: I66a08a3ad19a175b52737a5c3a81d3b1a5a786f5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/99702
Reviewed-by: Austin Eng <enga@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/fuzzers/BUILD.gn

@@ -85,6 +85,10 @@ if (build_with_chromium) {
       "tint_reader_writer_fuzzer.h",
       "transform_builder.h",
     ]
+
+    if (is_mac) {
+      sources += [ "//testing/libfuzzer/libfuzzer_exports.h" ]
+    }
   }
 
   source_set("tint_fuzzer_common_with_init_src") {