MdeModulePkg/BootMaintenanceManagerUiLib: hide library-internal symbol

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>