Add return to VulkanWindow::GetGraphicsDriverVendor() to suppress warning.

This does need to be properly implemented, however.

src/platform/linux/VulkanWindow.cpp

@@ -239,7 +239,7 @@ void VulkanWindow::SetVsyncEnabled(bool state) {
 }
 
 std::string VulkanWindow::GetGraphicsDriverVendor() {
-
+    return "Unknown OpenGL";// TODO: Implement properly!
 }
 
 VulkanWindow::VulkanWindow(const std::string& title, int width, int height) : RWindow(title, width, height) {