.gitignore: Ignore *-timestamp

Timestamp files were recently added to reduce make churn on source files
that use tracing.  The timestamp files should never be committed and
should not be visible in git status.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

.gitignore

@@ -4,6 +4,7 @@ config-host.*
 config-target.*
 trace.h
 trace.c
+*-timestamp
 *-softmmu
 *-darwin-user
 *-linux-user