openssh: add backported header file include

Backport upstream patch to add a missing header.  The patch says it is
for systemd, but I am seeing build issues when building openssh with
clang and musl.  The issue being seen is:
 #warning usage of non-standard #include <sys/cdefs.h> is deprecated
And similar deprecated warnings.  This patch resolves the issue.

Original patch can be found at
https://github.com/openssh/openssh-portable/commit/88351eca17dcc55189991ba60e50819b6d4193c1

This issue was introduced with OE-Core 1c9d3c22
https://github.com/openembedded/openembedded-core/commit/1c9d3c22718bf49ae85c2d06e0ee60ebdc2fd0c1



Patch suggested by Khem Raj.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Steve Sakoman <steve@sakoman.com>