aac: fix build with hardcoded tables

aac_tablegen.h includes aac.h for the POW_SF2_ZERO definition, but
this also pulls in a raft of other headers, some of which are not
safe to use in code built with the host compiler.

Moving POW_SF2_ZERO to aac_tablegen_decl.h, where the declaration
of the array it relates to already resides, fixes the problems.

Signed-off-by: Mans Rullgard <mans@mansr.com>