Don't use thin archives on OpenBSD 

OpenBSD's binutils doesn't support thin archives. The make generator 
backend already knows to avoid using alink_thin on OpenBSD, so make 
the same change to the ninja generator now that ninja supports 
OpenBSD. 

BUG=
Review URL: https://codereview.chromium.org/14913004/

Patch from mdempsky@google.com!



git-svn-id: http://gyp.googlecode.com/svn/trunk@1629 78cadc50-ecff-11dd-a971-7dbc132099af