BT CTS fails to build with some libCs

[aescolar]

Describe the bug
The BT CTS service fails to build with some libCs

To Reproduce
Steps to reproduce the behavior:

1. mkdir build; cd build
2. cmake -GNinja -DBOARD=nrf52_bsim ../samples/bluetooth/peripheral/
3. ninja
4. See error

Expected behavior
No build failures

Impact
Samples fails to build for some targets or with some libCs

Logs and console output

[97/247] Building C object zephyr/CMakeFiles/zephyr.dir/subsys/bluetooth/services/cts.c.obj
FAILED: zephyr/CMakeFiles/zephyr.dir/subsys/bluetooth/services/cts.c.obj
[..]
zephyr.dir/subsys/bluetooth/services/cts.c.obj -c subsys/bluetooth/services/cts.c
In file included from /usr/include/sys/types.h:130,
                 from include/zephyr/bluetooth/gatt.h:23,
                 from subsys/bluetooth/services/cts.c:15:
/usr/include/bits/types/timer_t.h:7:19: error: conflicting types for ‘timer_t’; have ‘__timer_t’ {aka ‘void *’}
    7 | typedef __timer_t timer_t;
      |                   ^~~~~~~
In file included from include/zephyr/posix/time.h:61,
                 from subsys/bluetooth/services/cts.c:12:
include/zephyr/posix/posix_types.h:93:23: note: previous declaration of ‘timer_t’ with type ‘timer_t’ {aka ‘long unsigned int’}
   93 | typedef unsigned long timer_t;
      |                       ^~~~~~~
subsys/bluetooth/services/cts.c: In function ‘bt_cts_time_from_unix_ms’:
subsys/bluetooth/services/cts.c:74:13: warning: implicit declaration of function ‘gmtime_r’; did you mean ‘gmtime’? [-Wimplicit-function-declaration]
   74 |         if (gmtime_r(&unix_ts, &date_time) == NULL) {
      |             ^~~~~~~~
      |             gmtime
subsys/bluetooth/services/cts.c:74:44: warning: comparison between pointer and integer
   74 |         if (gmtime_r(&unix_ts, &date_time) == NULL) {
      |                                            ^~

Environment (please complete the following information):

• OS: Ubuntu Linux 24.04
• Toolchain (e.g Zephyr SDK, ...): 0.17 or host gcc 14.2
• Commit SHA or Version used: 8dc3f85

Additional context
Related to #81412
Service introduced in 957d598