osdep: Fix compilation failure on BSD systems

osdep: Fix compilation failure on BSD systems

Fix compilation failure on BSD systems (which don't have
O_DIRECT or O_NOATIME:
osdep.c:116: error: ‘O_DIRECT’ undeclared (first use in this function)
osdep.c:116: error: (Each undeclared identifier is reported only once
osdep.c:116: error: for each function it appears in.)
osdep.c:116: error: ‘O_NOATIME’ undeclared (first use in this function)

Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>