Follow-up r308602: Don't add missing headers to .depend.tables.h.

This also avoids an error from egrep when a header is missing. This can happen
with something like WITHOUT_BLUETOOTH set when searching for
$include_dir/netgraph/bluetooth/include/ng_btsocket.h. The warning was
not an error (from set -e) due to being on the left side of a pipe. Now the
all_headers list is only filled with existing headers.

Reviewed by: ngie
MFC after: 3 days
Sponsored by: Dell EMC Isilon