bootstrap: chdir to bootstrap's directory

When integrating ninja into luvit I naively tried calling:

    ./tools/ninja/bootstrap.py

This broke because bootstrap expects you to be in the ninja directory
when you call it. This patch makes it possible to run bootstrap.py like
above and have it work.

bootstrap.py

@@ -21,6 +21,8 @@
 import shlex
 import subprocess
 
+os.chdir(os.path.dirname(sys.argv[0]))
+
 parser = OptionParser()
 parser.add_option('--verbose', action='store_true',
                   help='enable verbose build',)