gitignore: add entity-gen.cpp
gitignore: add entity-gen.cpp

Add generated file entity-gen.cpp

Change-Id: Id2d13de766b3d4c924bc9a48963e556a9ec86f58
Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>

openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid