Makefile.am: Fix build hang when .git and Git are present but don't work.

If the .git directory is there, and "git --version" works, but "git
ls-files" fails to print any file names (e.g. perhaps because .git is a
symlink to an inaccessible directory) then the "grep" command in the
thread-safety-check rule would hang forever because it had an empty list
of files and therefore expected to read from stdin.  This fixes the problem
by adding the name of an empty file to the command, which is harmless.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>