timer: rename NSEC_PER_SEC due to Mac OS X header clash

timer: rename NSEC_PER_SEC due to Mac OS X header clash

Commit e0cf11f31c24cfb17f44ed46c254d84c78e7f6e9 ("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 <[email protected]>
Message-id: 1436364609[email protected]
Signed-off-by: Peter Maydell <[email protected]>