error in samples/subsys/usb/cdc_acm

[holgerschurig]

Describe the bug
The CDC ACM sample isn't usable on native_posix. It pulls needlessly GPIO drivers in, without using them.

To Reproduce

1. cd samples/subsys/usb/cdc_acm
2. west build --pristine -b native_posix
3. Error at compilation time:

-- Configuring done
CMake Error at ../../../../cmake/extensions.cmake:398 (add_library):
  No SOURCES given to target: drivers__gpio

Expected behavior
I expected that the compilation actually worked, as USB seems to be emulated when compiled for native_posix and can be used via the Linux "usbip" command line tool.

BTW, removing the offending line fixes the issue:

1. sed -i '/CONFIG_GPIO/d' prj.conf
2. west build --pristine -b native_posix

Impact

Unable to test the sample in native posix mode.

Environment (please complete the following information):

• OS: Linux
• git describe --tags: zephyr-v2.4.0-388-g5bda586c64 (which is git HEAD as of now)

[github-actions]

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