[Windows] Fix dllimport issue in unit tests (flutter#35246)

In flutter/engine#35106, I landed a parameter type fix for the
declaration of FlutterDesktopEngineGetTextureRegistrar in our public
Windows C API. I also added a unit test that called this function.

That function (and all others) in our public Windows API is marked
FLUTTER_EXPORT, which resolves to __declspec(dllexport) or
__declspec(dllimport) depending on whether FLUTTER_DESKTOP_LIBRARY is
defined. It can be defined by adding the following build config:
//flutter/shell/platform/common:desktop_library_implementation

If the function is marked as an import, we get linker warnings that
we're importing a function that's defined in the same executable image.
This patch resolves this by marking the functions as export.

An alternative fix would be to support a third macro resolution that
resolves to nothing in the presence of some definition like
FLUTTER_NO_EXPORT; however, since flutter_export.h is a public header,
I'd prefer not to complicate it further, and this is a unit test that
can't be linked against either way.

Issue: flutter#109184
Original issue: flutter#86617

See: https://github.com/flutter/engine/blob/main/shell/platform/common/public/flutter_export.h
See: https://github.com/flutter/engine/blob/a51c7638e702b086dffaae3ce92f52130a2c23ff/shell/platform/common/BUILD.gn#L8-L10

No new tests since this simply fixes a link warning message in unit tests.

shell/platform/windows/BUILD.gn

@@ -205,6 +205,9 @@ executable("flutter_windows_unittests") {
     "window_unittests.cc",
   ]
 
+  configs +=
+      [ "//flutter/shell/platform/common:desktop_library_implementation" ]
+
   public_configs = [ "//flutter:config" ]
 
   deps = [