Use /MD for Windows builds.

I think the default is /MT, static linking.
This should make our builds smaller, and compatible with clients using /MD.

Change-Id: Id8a39a029925eda2627532bbd0223c693300f5da
Reviewed-on: https://skia-review.googlesource.com/5277
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gn/BUILD.gn

@@ -64,6 +64,7 @@ config("default") {
       "/FS",  # Preserve previous PDB behavior.
       "/bigobj",  # Some of our files are bigger than the regular limits.
       "/WX",  # Treat warnings as errors.
+      "/MD",  # Link against dynamic, non-debug standard library.
     ]
     defines += [
       "_CRT_SECURE_NO_WARNINGS",  # Disables warnings about sscanf().