Compile fuzzer sources in standalone builds.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index a54f6e401870960f8b3f6bd20d8a2be8661859f9..48b85c16b81112789136b796421ff10fe0e24c8b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1759,4 +1759,10 @@ if (pdf_is_standalone) {
       "//samples",
     ]
   }
+  group("fuzzers") {
+    testonly = true
+    deps = [
+      "//testing/libfuzzer",
+    ]
+  }
 }