cdc_acm: fix C++ issues with header and document dependency
cdc_acm: fix C++ issues with header and document dependency

The header was missing the language linkage wrapper that allows the
driver-specific function to be located from C++.

Also it helps the poor user to be informed that the function is only
available when a special Kconfig option is set.

Signed-off-by: Peter A. Bigot <650538893330be855f76604594b4820eb226769e@pabigot.com>

nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr