Fix compilation on UWP

It appears that MSVC and Mingw disagree about what conversions are legal
here. We just use the 'wide' version of the call and use our native
CharType instead of TCHAR.

drivers/windows/dir_access_windows.cpp

@@ -354,13 +354,13 @@ String DirAccessWindows::get_filesystem_type() const {
 	String unit = path.substr(0,unit_end+1) + "\\";
 	print_line("unit: "+unit);
 
-	TCHAR szVolumeName[100]    = "";
-	TCHAR szFileSystemName[10] = "";
+	WCHAR szVolumeName[100];
+	WCHAR szFileSystemName[10];
 	DWORD dwSerialNumber       = 0;
 	DWORD dwMaxFileNameLength  = 0;
 	DWORD dwFileSystemFlags    = 0;
 
-	if(::GetVolumeInformation(unit.utf8().get_data(),
+	if(::GetVolumeInformationW(unit.c_str(),
 				    szVolumeName,
 				    sizeof(szVolumeName),
 				    &dwSerialNumber,