Move vp8_strict_quantize_b inside EXACT_QUANT #define.

There is currently no inexact version of this function, so do not
 even compile it without EXACT_QUANT.
This will prevent someone from inadvertently trying to use it without
 the proper EXACT_QUANT setup.

Change-Id: Ia13491e0128afb281c05c9222ee5987101e4010d