GN: _turbo -> -turbo to match Fuchsia

We might as well match the folks who are using our GN files now.

We've got plenty of strategies in our pocket for when we try to move Chrome
onto our GN files (and who knows, there may be even a new better way by then):
  * Same sort of rename in Chrome's third_party
  * Aliased targets via //build/secondary in Chrome.
  * Indirection via build_overrides

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2265503002

Review-Url: https://codereview.chromium.org/2265503002

BUILD.gn

@@ -190,7 +190,7 @@ component("skia") {
   deps = [
     "//third_party/expat",
     "//third_party/giflib",
-    "//third_party/libjpeg_turbo:libjpeg",
+    "//third_party/libjpeg-turbo:libjpeg",
     "//third_party/libpng",
     "//third_party/libwebp",
     "//third_party/sfntly",