sysutils/safecat: avoid duplicating object files in library

Previously byte_copy.o and str_len.o were added to str.a, and then
specified as objects on the safecat link command line along with str.
When linking with lld this produces duplicate symbol errors like:

/usr/bin/ld: error: duplicate symbol: byte_copy

defined at byte_copy.c

byte_copy.o:(byte_copy) in archive str.a

defined at byte_copy.c

byte_copy.o(.text+0x0)

As these symbols are provided by str.a just avoid listing the object
files as well.

PR: 221811
Approved by: matthew
Sponsored by: The FreeBSD Foundation