MdeModulePkg Variable: Move "extern XXX" to Variable.h

MdeModulePkg Variable: Move "extern XXX" to Variable.h

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.

Move "extern XXX" to Variable.h from VariableDxe.c/VariableSmm.c.

This patch prepares for adding emulated variable NV mode
support in VariableRuntimeDxe.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>