tools/usb: Add .gitignore file

Add .gitignore file for untracked files in tools/usb.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

tools/usb/.gitignore

@@ -0,0 +1,2 @@
+ffs-test
+testusb