bionic: add macro protection for MALLOC_ALIGNMENT

In previous commit: 2fd81ef7, .mk file has been modified to
configure MALLOC_ALIGNMENT dynamicly according to board config.
Add the missing macor protection here.

Change-Id: I703cca2ce0504ab3e11aab226b2c61fcc0c6afa1
Author: Jin Wei <wei.a.jin@intel.com>
Signed-off-by: Xiaokang Qin <xiaokang.qin@intel.com>
Signed-off-by: Beare, Bruce J <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>