Fix mingw compiler error due to missing function declaration (google#297

)

This fixes the following compiler errors.
time_zone_lookup.cc:78:34: error: 'WindowsCreateStringReference' was not declared in this scope
time_zone_lookup.cc:84:34: error: 'WindowsDeleteString' was not declared in this scope
time_zone_lookup.cc:90:34: error: 'WindowsGetStringRawBuffer' was not declared in this scope

Also, official documentation suggests to include the winstring.h header.
https://learn.microsoft.com/en-us/windows/win32/api/winstring/nf-winstring-windowsgetstringrawbuffer

src/time_zone_lookup.cc

@@ -45,6 +45,7 @@
 #include <wchar.h>
 #include <windows.globalization.h>
 #include <windows.h>
+#include <winstring.h>
 #endif
 #endif