util/cbfstool/compiler: __attribute redefinition

In Windows Cygwin enviroment, compiler reports
redefinition error at cbfstool/ifwitool.c
on _packed and __aligned.

Skip new defines when vales are already defined.

Change-Id: I3af3c6b8fc57eee345afcef2f871b897138f78ce
Signed-off-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-on: https://review.coreboot.org/27357
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
1 file changed