tools/mkimage: backport gcc5 compatibility, patch

tools/mkimage: backport gcc5 compatibility, patch

Fedora 22 includes gcc5 by default
build leads to error:
include/linux/compiler-gcc.h:114:30: fatal error: linux/compiler-gcc5.h: No such file or directory

backport upstream patch: "Add linux/compiler-gcc5.h to fix builds with gcc5"
id: 478b02f1a7043b673565075ea5016376f3293b23

Issue was reported at
https://forum.openwrt.org/viewtopic.php?id=56853
& replicated in a VM

created, flashed and tested ar71xx image successfully
as test

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45703 3c298f89-4303-0410-b956-a3cf2f4a3e73