postfix: fix clean broken issue

There is a build failure while rebuilding postfix
...
| NOTE: make -e MAKEFLAGS= OPT= DEBUG= OPTS= clean
| make -f Makefile.in MAKELEVEL= Makefiles
| (echo "# Do not edit -- this file documents how Postfix was built for your
machine.";/bin/sh makedefs) >makedefs.tmp
| No <db.h> include file found.
| Install the appropriate db*-devel package first.
| Makefile.in:31: recipe for target 'Makefiles' failed
| make: *** [Makefiles] Error 1
| Makefile:21: recipe for target 'Makefiles' failed
| make: *** [Makefiles] Error 2
| ERROR: oe_runmake failed
| ERROR: Function failed: do_configure (log file is located at tmp/work/
core2-64-wrs-linux/postfix/3.0.2-r0/temp/do_configure/log.do_configure.12848)
...

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>