__assert.h: format strings are not optional

If the reason strings were omitted, the build was only
failing if assertions were turned on, in printk().
Now it fails regardless of whether they are turned on.

Change-Id: I90721a5babb89b20bf8430e122eb03a4ec62c46f
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>