top .gitignore: ignore .dirstamp files

Just after succesfull build you can see some untracked files
in the 'git status' output:

	src/jtag/drivers/versaloon/.dirstamp
	src/jtag/drivers/versaloon/usbtoxxx/.dirstamp

This commit fixes the problem.

Change-Id: I1674eb4423e97c5f0a47f216981eaffdc351f784
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Reviewed-on: http://openocd.zylin.com/1898


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

.gitignore

@@ -3,6 +3,7 @@
 # build output
 .libs
 .deps
+.dirstamp
 *.o
 *.o.??????
 *.a