sreadahead: adding readahead system call into bionic libc

Add bionic libc to support readahead system call.
This is needed to enable sreadahead to work.

Change-Id: I3856e1a3833db82e6cf42fd34af7631bd40cc723
Author: Winson Yung <winson.w.yung@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>

libc/include/sys/linux-unistd.h

@@ -20,6 +20,7 @@ gid_t            getegid (void);
 uid_t            getresuid (void);
 gid_t            getresgid (void);
 pid_t            gettid (void);
+ssize_t          readahead (int, off64_t, size_t);
 int              getgroups (int, gid_t *);
 pid_t            getpgid (pid_t);
 pid_t            getppid (void);