util: Move definition of HANDLE here.

A few Open vSwitch source files use a type named HANDLE on Windows systems,
in a way that makes it easier to avoid #ifdefs if we have a dummy
definition on other platforms.  <linux/types.h> was a really weird place
for this dummy definition.  This commit moves it to util.h.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>