MdeModulePkg/BootMaintenanceManagerUiLib: hide library-internal symbol
    
    Static storage duration objects that are internal to a library instance
    should:
    - either have internal linkage (i.e., be declared STATIC),
    - or, if they are referenced in multiple files of the library instance,
      prefixed with a word that is specific to the library instance, and
      minimizes namespace collisions.
    
    In this case, the "gHiiDriverList" variable (with static storage duration
    and external linkage) is defined in both BootMaintenanceManagerUiLib and
    UiApp. When these are linked together, GCC catches the multiple external
    definitions and aborts the build. (GCC notices this due to commit
    214a3b79417f.) Fix the error by applying the first rule above.
    
    Fixes: a85be3ae48a8aaa40b755cd0ff7270c67cfed585
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>
    Reviewed-by: Eric Dong <eric.dong@intel.com>