libseccomp: install missing header file

As of version 2.4.2, libseccomp ships a new header file
seccomp-syscalls.h. Install it in InstallDev.

Fixes: 71b663b3

 ("libseccomp: update to version 2.4.2")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Rosen Penev <rosenp@gmail.com>