cmake: fix libusb search

Adjust cmake test for libusb library so that the compatible libusb for
freebsd can be found. Since freebsd's libusb header are located in
a different place from libusb-1.0, the includes in the source code must
be adjusted using #include <libusb.h>. Actually those new include
statements are those that libusb-1.0 really expects to be used (see
libusb-1.0.pc cflags to verify this).

Taken from libfreenect commit 8219750df3a13501626a7f7f54d24afb8e64ec3f
https://github.com/OpenKinect/libfreenect/commit/8219750df3a13501626a7f7f54d24afb8e64ec3f