MdeModulePkg/VariableSmmRuntimeDxe.inf: list local header "Variable.h"

MdeModulePkg/VariableSmmRuntimeDxe.inf: list local header "Variable.h"

Commit aab3b9b9a1e5 ("MdeModulePkg/Variable: Add RT GetVariable() cache
support", 2019-11-05) added "VariableParsing.h" to
"VariableSmmRuntimeDxe.inf".

"VariableParsing.h" includes "Variable.h", so the "build" utility is right
to warn us that "Variable.h" should be listed in [Sources] too.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kubacki <michael.a.kubacki@intel.com>
Fixes: aab3b9b9a1e5e1f3fa966fb1667fc3e6c47e7706
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>