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 ..