OvmfPkg: Undo removal of License.txt

OvmfPkg: Undo removal of License.txt

Undo removal of OvmfPkg/License.txt in commit

https://github.com/tianocore/edk2/commit/2a98de03444e6a1e04030e70bb85e62de6b82edc

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Andrew Fish <afish@apple.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>