drivers/cc110x: add xtimer as module dependency

[smlng]

adds missing dependency, this popped up during test of #5469 and #6453 respectively when compiling the kw2xrf driver test for board msba2 with cc110x. The latter needs xtimer, linker error without this fix:

/RIOT/tests/driver_kw2xrf/bin/msba2/cc110x.a(cc110x-spi.o): In function `_xtimer_tsleep32':
/RIOT/sys/include/xtimer/implementation.h:147: undefined reference to `_xtimer_tsleep'
/RIOT/tests/driver_kw2xrf/bin/msba2/cc110x.a(cc110x.o): In function `_xtimer_tsleep32':
/RIOT/sys/include/xtimer/implementation.h:147: undefined reference to `_xtimer_tsleep'
/RIOT/sys/include/xtimer/implementation.h:147: undefined reference to `_xtimer_tsleep'
collect2: error: ld returned 1 exit status
make: *** [all] Error 1

[PeterKietzmann]

ACK