Fix MISRA rule 8.8 in common code

Rule 8.8: The static storage class specifier shall be used
          in all declarations of objects and functions that
          have internal linkage.

Change-Id: I1e94371caaadebb2cec38d0ae0fa5c59e43369e0
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>