libusb.h: Simplify condition governing the inclusion of sys/time.h

libusb.h: Simplify condition governing the inclusion of sys/time.h

The expression for the condition grows each time a new backend is added,
but it can be simplified if changed to key off of the _MSC_VER macro.
The sys/time.h header is ubiquitous across all platforms and build
environments except Visual Studio, so change the condition to be based
on this. This fixes builds on BSD where the condition was not evaluating
to true.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>