Fixed a compilation warning on winapi.cpp

Fixes #2690

Thanks to @hurtsky, @reymarbc and @keitenai

src/common/winapi.cpp

@@ -161,7 +161,7 @@ bool IsCurrentUserLocalAdministrator(void){
 		if (hToken) CloseHandle(hToken);
 	}
 
-	return fReturn;
+	return fReturn == TRUE;
 #else
 	return false;
 #endif