avcodec/simple_idct: use predefined M_SQRT2

M_SQRT2 is defined in math.h, or in avutil/mathematics.h for
compatibility hacks. This uses this value instead of a floating literal.

Fixed point values produced by C_FIX(), R_FIX() remain identical.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
