package/kexec: Fix musl build issue related to loff_t

Fixes
http://autobuild.buildroot.net/results/c19/c19a016b7fd50b8cef110e70e6f3af7cd0369368/



[Peter: extend commit message]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>