sys/ztimer: fix backend selection

[leandrolanzieri]

Contribution description

With the introduction of ztimer_no_periph_rtt in #17284 ztimer_msec and ztimer_sec don't fallback to ztimer_periph_timer when the pseudomodule is active. This adds the fallback selection again.

Testing procedure

• Something like BOARD=samr21-xpro USEMODULE="ztimer_msec ztimer_no_periph_rtt" make -C examples/hello-world/ should work with this PR, but fails on master due to lack of backend selection.

Issues/PRs references

#17284

[kfessel]

Ad: with #16342 this issue would have had a nicer error message

[kfessel]

#16342 would had give you a nice message in case of a missing timer backend

read change ✔️
test master bug verified ✔️
test pr bug gone ✔️

[kfessel]

with #16342

RIOT-master/sys/ztimer/init.c:182:6: error: #error No suitable ZTIMER_MSEC config. No rtt or basic timer?
  182 | #    error No suitable ZTIMER_MSEC config. No rtt or basic timer?
      |      ^~~~~
RIOT-master/sys/ztimer/init.c:188:38: error: ‘ZTIMER_RTT_CLK’ undeclared here (not in a function)
  188 | ztimer_clock_t *const ZTIMER_MSEC = &ZTIMER_RTT_CLK;