OvmfPkg: QemuFwCfgLib: avoid "variable set but not used" warning from GCC

OvmfPkg: QemuFwCfgLib: avoid "variable set but not used" warning from GCC

The FileReserved variable in QemuFwCfgFindFile() is only used to skip
over the reserved field in file headers, which causes newer versions of
GCC to flag it with a "variable set but not used" warning (which is normally
not visible since as of right now these warnings are supressed). It's true
that the value read into FileReserved is never used, but this is
intentional. This patch adds a do-nothing reference to silence the
warning.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Bill Paul <wpaul@windriver.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17920 6f19259b-4bc3-4df7-8a09-765794883524