stagefright amrwb: Change a C++ style cast into a C style cast

This fixes compilation as C - while the source files are named
.cpp, they actually are compileable as pure C (except for this
cast).

This was committed on the master branch of the opencore
repository in commit ae27d9b2ef68c4e6fc383a8cab93d6baa235ff6b,
based on AOSP contribution 10014.

Change-Id: I4f533313be041ad3e0a03ffc5f4207480894ddd1