samples: net: gsm_modem: build fails if CONFIG_GSM_MUX=y

[hwilmers]

Describe the bug
Building of sample app gsm_modem fails if GSM muxing is enabled
by setting CONFIG_GSM_MUX=y.

To Reproduce
Steps to reproduce the behavior:

1. cd samples/net/gsm_modem
2. west build -b reel_board
   -> OK, builds fine.

Now append to prj.conf:
CONFIG_GSM_MUX=y

Repeat build:

1. west build
   -> build fails

Expected behavior
Build should succeed.

Impact
Showstopper - GSM muxing can not be used.

Logs and console output
...
../../../../include/sys/cbprintf_internal.h:183:19: error: invalid use of undefined type 'struct gsm_dlci'
183 | *_Generic((arg) + 0,
...

Environment (please complete the following information):

• OS: Linux
• Toolchain: Zephyr SDK 0.11.2
• Commit: zephyr-v2.5.0-3633-geaa2dba1a232

[galak]

@hwilmers this appears fixed on the main tree.

[hwilmers]

Someone is working faster than me. Thanks ;-)