[libfuzzer] use source-built expat parser

third_party/expat/BUILD.gn

Index: third_party/expat/BUILD.gn
diff --git a/third_party/expat/BUILD.gn b/third_party/expat/BUILD.gn
index e70f31be067f4c0562daa86f364b1048e843d29f..642cd7ac2ddf2783cd724cbc0f0253e8847d735d 100644
--- a/third_party/expat/BUILD.gn
+++ b/third_party/expat/BUILD.gn
@@ -5,8 +5,11 @@
 import("//build/config/chromecast_build.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 
-# Chromecast doesn't ship expat as a system library
-if (is_linux && !is_chromecast) {
+# On Linux, we implicitly already depend on expat via fontconfig;
+# let's not pull it in twice.
+# Chromecast doesn't ship expat as a system library.
+# Libfuzzer needs to build library from sources.
+if (is_linux && !is_chromecast && !use_libfuzzer) {
   config("expat_config") {
     libs = [ "expat" ]
   }