postfix.inc: Remove references to buildmachine paths in target makedefs.out

Fixed the buildpaths QA issue:
ERROR: QA Issue: File
/work/core2-64-wrs-linux/postfix/2.11.1-r0/packages-split/postfix/etc/postfix/makedefs.out
in package contained reference to tmpdir [buildpaths]

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>