sig_pri.h: force_restart_unavailable_chans in wrong scope

In channels/sig_pri.h, struct sig_pri_span, the field
force_restart_unavailable_chans is only defined if

#if defined(HAVE_PRI_MCID) is true.

All other occurences of force_restart_unavailable_chans are outside of the

#if defined(HAVE_PRI_MCID)
endif

scope.

ASTERISK-25257 #close
Reported by: Patric Marschall

Change-Id: I071de89cc2cd0d85927a013036e235851f672549