efi_selftest: update .gitignore

The following generated files should be ignored by git:
efi_miniapp_file_image_exit.h
efi_miniapp_file_image_return.h

*.so files are normally deleted during the build but should be
ignored too.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>