darwinssl: Fix #if 10.6.0 for SecKeychainSearch

The comment here says that SecKeychainSearch causes a deprecation
warning when used with a minimum Mac OS X SDK version of 10.7.0, which
is correct.  However, the #if guard did not match.  It was intended to
only use the code if 10.6.0 support was enabled, but it had 10.7.0
instead.  This caused a warning if the minimum was exactly 10.7.0.