make exec_ops static

While working on some target stack changes, I noticed that exec_ops is
only used from exec.c.  This patch makes it "static".  This is cleaner
and makes it simpler to reason about the use of the target.

Tested by rebuilding.
I'm checking this in as obvious.

2014-07-18  Tom Tromey  <tromey@redhat.com>

	* exec.c (exec_ops): Now static.
	* exec.h (exec_ops): Don't declare.