Fix OSS-Fuzz build.

-Wimplicit-int-float-conversion seems enabled per default for OSSFuzz's
clang. Explicitly disable so our build succeeds.

Bug: 140221808
Change-Id: I30cfe185bdf28f4e447109d24e0572a38cd322d6
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 6ac8191..6e2afb4 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -285,7 +285,7 @@
 }
 
 # Used by fuzzers.
-if (enable_perfetto_fuzzers) {
+if (enable_perfetto_fuzzers && use_libfuzzer) {
   group("libfuzzer") {
     assert(perfetto_root_path == "//")
     public_deps = [