core: include sys/time.h on any GNU libc OS

core: include sys/time.h on any GNU libc OS

GNU libc provides sys/time.h, so unconditionally include it when using
that libc.

Since sys/time.h is already included on a number of OSes, include it to
minimize the differences between OSes when using libusb. Arguably,
sources using functions from sys/time.h (such as gettimeofday) ought to
already include it on their own; OTOH, let's avoid making such issues
OS-specific.