ANDROID: virtio_balloon: Rename header guard in uapi header file

This will allow introduction of a Linux-private header file with the
usual guard name, which would currently conflict with this uapi
header.

Bug: 381400679
Bug: 357781595
Ignore-Checkpatch: Trivial change to a uapi header file with no SPDX line
Change-Id: I0a4f949cfb539becab2f706c04562aecd16866b3
Signed-off-by: Keir Fraser <keirf@google.com>