OvmfPkg/Csm/CsmSupportLib: fix "missing braces around initializer"

Recent BaseTools changes trigger this gcc warning.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/OvmfPkg@15726 6f19259b-4bc3-4df7-8a09-765794883524