Ignore temporary object files

Prior to this change, `git status` would report untracked files of the 
following sort if run during a build:

    ?? src/rpcprotocol.o-e628def3

These files should be explicitly ignored not only because they are a 
nuisance, but given that they appear and disappear quickly, they may be 
inadvertently added to the index even if one has been careful to check for
untracked files with `git status` prior to a `git add .`.