Cast ::GetLastError to int (flutter#57113)

`GetLastError` returns an unsigned 32 bit integer that was being
implicitly cast to an int for the std::variant<..., int>. This was
causing my build to fail with:

```
../../flutter/shell/platform/windows/platform_handler.cc(178,12): error: no viable conversion from returned value of type 'DWORD' (aka 'unsigned long') to function return type 'std::variant<std::wstring, int>' (aka 'variant<basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t>>, int>')
  178 |     return ::GetLastError();
      |            ^~~~~~~~~~~~~~~~
../../../../../../../Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.42.34433/include\variant(923,7): note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'DWORD' (aka 'unsigned long') to 'const variant<basic_string<wchar_t>, int> &' for 1st argument
  923 | class variant : private _SMF_control<_Variant_destroy_layer<_Types...>, _Types...> { // discriminated union
      |       ^~~~~~~
```

Commands:

```
 ./flutter/tools/gn --runtime-mode release --no-rbe
ninja -C .\out\host_release windows gen_snapshot flutter/build/archives:windows_flutter
```

Explicitly casting `::GetLastError` to an int fixes this issue.

I'm running on Windows 11 (Version 10.0.26100 Build 26100) with VS 2022
Community Edition.

@loic-sharma

Co-authored-by: Eric Seidel <eric@shorebird.dev>

Author: bryanoltman

shell/platform/windows/platform_handler.cc

@@ -60,15 +60,15 @@ class ScopedGlobalMemory {
     memory_ = ::GlobalAlloc(flags, bytes);
     if (!memory_) {
       FML_LOG(ERROR) << "Unable to allocate global memory: "
-                     << ::GetLastError();
+                     << static_cast<int>(::GetLastError());
     }
   }
 
   ~ScopedGlobalMemory() {
     if (memory_) {
       if (::GlobalFree(memory_) != nullptr) {
         FML_LOG(ERROR) << "Failed to free global allocation: "
-                       << ::GetLastError();
+                       << static_cast<int>(::GetLastError());
       }
     }
   }
@@ -175,12 +175,12 @@ std::variant<std::wstring, int> ScopedClipboard::GetString() {
 
   HANDLE data = ::GetClipboardData(CF_UNICODETEXT);
   if (data == nullptr) {
-    return ::GetLastError();
+    return static_cast<int>(::GetLastError());
   }
   ScopedGlobalLock locked_data(data);
 
   if (!locked_data.get()) {
-    return ::GetLastError();
+    return static_cast<int>(::GetLastError());
   }
   return static_cast<wchar_t*>(locked_data.get());
 }