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 .`.

.gitignore

@@ -34,6 +34,7 @@ src/qt/test/moc*.cpp
 *.rej
 *.orig
 *.o
+*.o-*
 *.patch
 .bitcoin
 *.a