GN: Define SK_HAS_... in all Skia-private code, not just libskia.

SkForceLinking wasn't force-linking the WEBP encoder, or any encoder.
(The others must be explicitly used.)

This should help the GN Debug Perf bots progress past

    Running Encode_color_wheel.jpg_WEBP nonrendering
    ../../../bench/EncoderBench.cpp:58: fatal error: "assert(data)"

On my laptop, a debug nanobench run now finishes.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2196243002

Review-Url: https://codereview.chromium.org/2196243002
diff --git a/BUILD.gn b/BUILD.gn
index d6e2562..8c519d9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -56,6 +56,13 @@
     "third_party/etc1",
     "third_party/ktx",
   ]
+
+  defines = [
+    "SK_HAS_GIF_LIBRARY",
+    "SK_HAS_JPEG_LIBRARY",
+    "SK_HAS_PNG_LIBRARY",
+    "SK_HAS_WEBP_LIBRARY",
+  ]
 }
 
 # Any code that's linked into Skia-the-library should use this config via += skia_library_configs.
@@ -161,13 +168,6 @@
     "//third_party/zlib",
   ]
 
-  defines = [
-    "SK_HAS_GIF_LIBRARY",
-    "SK_HAS_JPEG_LIBRARY",
-    "SK_HAS_PNG_LIBRARY",
-    "SK_HAS_WEBP_LIBRARY",
-  ]
-
   libs = [ "pthread" ]
 
   sources = []