stagefright: Include stdlib.h for malloc/free

According to ANSI C, malloc/free are provided by stdlib.h.

Also, string.h is always available according to ANSI C, and provides
the memset function, so there's no need to include it conditionally
for linux only.

This makes the code buildable on OS X, too.

Change-Id: Ie5d1da02158797e7e45dea257cbe3e4996dde4db