dahdi_dummy: Build failure with kernel >= 6.15

[mnhauke]

hrtimer_init() was deprecated when hrtimer_setup() was added in Kernel
6.13. As of Kernel 6.15 hrtimer_init() has been removed.

context
torvalds/linux@908a1d7
torvalds/linux@9779489

build failure

dahdi_dummy.c: In function ‘init_module’:
dahdi_dummy.c:232:9: error: implicit declaration of function ‘hrtimer_init’; did you mean ‘hrtimers_init’? [-Wimplicit-function-declaration]
  232 |         hrtimer_init(&zaptimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
      |         ^~~~~~~~~~~~
      |         hrtimers_init
make[4]: *** [/usr/src/linux-6.15.5-1/scripts/Makefile.build:204: dahdi_dummy.o] Error 1