timer: rename NSEC_PER_SEC due to Mac OS X header clash

Commit e0cf11f3 ("timer: Use a single
definition of NSEC_PER_SEC for the whole codebase") renamed
NANOSECONDS_PER_SECOND to NSEC_PER_SEC.

On Mac OS X there is a <dispatch/time.h> system header which also
defines NSEC_PER_SEC.  This causes compiler warnings.

Let's use the old name instead.  It's longer but it doesn't clash.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1436364609-7929-1-git-send-email-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>