Unicode: Fix mismatched type initialization

Summary:
Fixes the following:
system/core/libutils/Unicode.cpp:579:27: error: cannot initialize a variable of type 'const uint16_t *const' (aka 'const
unsigned short *const') with an rvalue of type 'char16_t *'
    const uint16_t* const u16end = dst + dstLen;
                          ^        ~~~~~~~~~~~~

Test Plan: verifying

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D117