Do not pass -Werror on fuzzers.

OSS-Fuzz change their build environment often enough for this to be
necessary.

Change-Id: Idb3c0d33ee4e36c98e1c01acd42a0bc3fdad4530
diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index f586a24..247e63a 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -76,9 +76,12 @@
     "-fPIC",
     "-g",
     "-Wformat",
-    "-Werror",
   ]
 
+  if (!is_fuzzer) {
+    cflags += [ "-Werror" ]
+  }
+
   if (is_clang) {
     cflags += [
       # Color compiler output, see https://github.com/ninja-build/ninja/wiki/FAQ