wafsamba: fix samba_abi for default catch-all case

wafsamba: fix samba_abi for default catch-all case

Only filter out the symbol when positive match was not found and there is
negative match.

ABI signature file generator worked incorrectly for cases when mixture of
positive and negative matches were provided. This resulted in generating empty
signature file for libpdb since there was no catch-all positive match anymore.

Commit 9ba44cc610426fb558b49aa9680b5bdf55c29082 removed explicit '*' positive
match and corresponding vscript generator adds '*' by default if global match
list is empty, so this commit introduces feature parity into signature
generator.

Reviewed-by: Andreas Schneider <asn@samba.org>