build-unit-test-docker: gpioplus requires stdplus
build-unit-test-docker: gpioplus requires stdplus

In anticipation of changes upcoming in gpioplus where it will use the
File Descriptor types provided by stdplus.

Change-Id: Ie682564b04e7f6799277301c5f4384b2b05524bf
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts