vp8l_enc.c: normalize index types
vp8l_enc.c: normalize index types

fixes conversion warnings in visual studio after:
b1674240 Add modified Zeng's method to palette sorting.

src\enc\vp8l_enc.c(296) : warning C4244: '=' : conversion from 'const
uint16_t' to 'uint8_t', possible loss of data
src\enc\vp8l_enc.c(299) : warning C4244: '=' : conversion from 'const
uint16_t' to 'uint8_t', possible loss of data

Change-Id: I981b1ba4912edbbafbd49f1f5b1043bf12266920

webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp