setfiles: fix build failure after common_bufsiz change

setfiles: fix build failure after common_bufsiz change

Commit 47cfbf32fd66563f8c4e09ad6cced6abfbe2fad5 ("*: add most of the
required setup_common_bufsiz() calls") switched this tool over to use
the common_bufsiz logic but missed including the header leading to a
build failure when enabled:
selinux/setfiles.c:80:30: error: 'bb_common_bufsiz1' undeclared (first use in this function)

URL: https://bugs.gentoo.org/600106
Reported-by: Jonas Jelten <jj@stusta.net>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>