stagefright amrwb: Make a global const array static

This fixes compilation as C - while the source files are named
.cpp, they actually are compileable as pure C. Prior to this,
this table produced multiple definitions of the same symbol.

This was committed on the master branch of the opencore
repository in commit a803bd49a631317e0d52ff060cf1799f7741a47e,
based on AOSP contribution 10017.

Change-Id: Id76274964242b119ab61b8181e557477df1e1521