OvmfPkg/IoMmuDxe: Fix header guard macro
OvmfPkg/IoMmuDxe: Fix header guard macro

Correct the header guard macro

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Thomas Palmer <60f4d02ff984b42bd1e65cdc2278843916e619c4@hpe.com>
Reviewed-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>

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