Add missing include in DawnWireServerAndFrontendFuzzer.cpp

This file was missing an include of libfuzzer_exports.h which was
causing LLVMFuzzerInitialize to be stripped from the binary.

Tbr=cwallez@chromium.org
Bug: chromium:1075605
Change-Id: I251e3dd7ad1b3ebb95339dec946b80ce2e5d4a05
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/21301
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/fuzzers/DawnWireServerAndFrontendFuzzer.cpp

@@ -16,6 +16,7 @@
 
 #include "common/Assert.h"
 #include "dawn_native/DawnNative.h"
+#include "testing/libfuzzer/libfuzzer_exports.h"
 
 extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv) {
     return DawnWireServerFuzzer::Initialize(argc, argv);