missing_syscall.h: include errno.h

This include is needed for errno and ENOSYS

Fixes:
 - http://autobuild.buildroot.org/results/699c078aa078240c6741da4dbd0871450ceeca92



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
(cherry picked from commit e2d94d0c)