Change shared_sources.gni to use relative path

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 7b506e4388e2c78ad463870dcc4a76c870954203..258bf2d226a228c0803d7bebfe7f6ee51b6b314d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -511,7 +511,10 @@ component("skia") {
   }
 
   if (is_fuchsia) {
-    sources += [ "src/gpu/gl/GrGLCreateNativeInterface_none.cpp" ]
+    sources += [
+      "src/gpu/gl/GrGLCreateNativeInterface_none.cpp",
+      "src/ports/SkDebug_stdio.cpp",
+    ]
   }
 }