avcodec/libilbc: support for latest git of libilbc

in the latest git commits of libilbc developers removed WebRtc_xxx typedefs

This commit uses int types instead,
it's safe to apply also for previous versions since
WebRtc_Word16 was always a typedef of int16_t and
WebRtc_UWord16 a typedef of uint16_t

Reviewed-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>