+ M_SQRT2 / M_SQRT1_2 to keep the daala diffs down

adapted from:
ebb9b28 Move math.h fills to odintrin.h.

these aren't visible by default with -std=c99.

BUG=aomedia:111

Change-Id: Iaa65986f35d914bf92c8c49a8211e0e6864c64e4