Compile fuzzer sources in standalone builds. (try 2)

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 9a8bf07f206789a7fa543731b69e56a84893b60b..1f73c125c703b721e215f75387cb1201d3eb5688 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1768,4 +1768,10 @@ if (pdf_is_standalone) {
       "//samples",
     ]
   }
+  group("fuzzers") {
+    testonly = true
+    deps = [
+      "//testing/libfuzzer",
+    ]
+  }
 }