syslog-ng: add gawk to RDEPENDS

This solves the following warning:

syslog-ng-3.5.4.1: syslog-ng requires /usr/bin/awk, but no providers in its
RDEPENDS [file-rdeps]

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>