syscallsubr.h: Header inclusions: Explicit <sys/types.h>, sort

<sys/types.h> is currently brought in by other headers but some of its
type definition are directly used in this header, so it should appear
explicitly. It is necessary as lots of prototypes in there use types it
defines (<sys/_types.h> wouldn't be enough).

Sort header inclusions as per style(9).

No functional change (intended).

Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47881