drs: rgb camera hook (#524)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Add plug&play hooks for RGB camera in DRS.

## Testing
Seems to work with my crappy Logitech C270

Camera options (brightness etc) seem to work in test menu as well).

Tested by another user with realsense camera + logitech camera, both
cams were detected correctly.

src/spice2x/games/drs/rgb_cam.h

@@ -0,0 +1,5 @@
+#pragma once
+
+namespace games::drs {
+    void init_rgb_camera_hook();
+}