Add missing idf cmake file.

Author: raldone01

examples/device/webusb_serial/src/CMakeLists.txt

@@ -0,0 +1,4 @@
+# This file is for ESP-IDF only
+idf_component_register(SRCS "main.c" "usb_descriptors.c"
+                    INCLUDE_DIRS "."
+                    REQUIRES boards tinyusb_src)