Add fuzz executable to GN

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2389853002

Review-Url: https://codereview.chromium.org/2389853002
diff --git a/BUILD.gn b/BUILD.gn
index 3a81d36..a215494 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -987,4 +987,20 @@
     ]
     testonly = true
   }
+
+  executable("fuzz") {
+    sources = [
+      "fuzz/FilterFuzz.cpp",
+      "fuzz/FuzzGradients.cpp",
+      "fuzz/FuzzParsePath.cpp",
+      "fuzz/FuzzPathop.cpp",
+      "fuzz/FuzzScaleToSides.cpp",
+      "fuzz/fuzz.cpp",
+    ]
+    deps = [
+      ":flags",
+      ":skia",
+    ]
+    testonly = true
+  }
 }