avcodec/ratecontrol: use predefined M_E

M_E is defined in math.h, or in avutil/mathematics.h for compatibility
hacks. This uses this value instead of an ad-hoc define.

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