Carry over r311520 to tools/build/options/WITHOUT_USB_GADGET_EXAMPLES

Discussed with: wblock

r311520:

Fix src.conf(5) description of WITHOUT_USB_GADGET_EXAMPLES.

PR: 215831
Submitted by: p5B2E9A8F@t-online.de
MFC after: 1 week
X-MFC with: r311520
Sponsored by: iXsystems