automake: add dummy.cc to fix 'make tags'

Commit 421e6c561704 ("test_librbd_fsx: add krbd mode support") added
a requirement for ceph_test_librbd_fsx to be linked with a C++ linker.
Implement it in a way that doesn't break 'make tags'.

Fixes: #8530
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>