Update NPC head yaw variable.

The correct variable for NPC head yaw is aP not aO or aN.
The changes went like this:
1.4.7-1.5
	az-aA
1.5.1-1.6.1
	aA-aP

In the future if you're unsure just look near the end of the constructor for EntityLiving.java for the line "this.[variable name] = this.yaw;"