thunderbolt: Fix compile-time warning

The Thunderbolt panel warns about explicit_bzero() not being available
but never checked for it.

gnome-control-center/panels/thunderbolt/bolt-str.c: In function ‘bolt_erase_n’:
gnome-control-center/panels/thunderbolt/bolt-str.c:33:4: warning: #warning no explicit bzero, using fallback [-Wcpp]
   #warning no explicit bzero, using fallback
    ^~~~~~~

Closes: #286
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