Add gs_device_name function

This returns the name of the device, "Direct3D 11" or "OpenGL"
respectively.

libobs/graphics/graphics-imports.c

@@ -40,6 +40,7 @@ bool load_graphics_imports(struct gs_exports *exports, void *module,
 {
 	bool success = true;
 
+	GRAPHICS_IMPORT(device_name);
 	GRAPHICS_IMPORT(device_preprocessor_name);
 	GRAPHICS_IMPORT(device_create);
 	GRAPHICS_IMPORT(device_destroy);