ShellPkg: fix "incompatible pointer type" build error

Following commit 7fe3fbad05cd6ef23e83ab5da8c5a82ff9bb456e, building
OvmfPkg on Linux breaks with an "incompatible pointer type" error.
This patch adds a cast to supress the warning and allow the build
to complete

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16198 6f19259b-4bc3-4df7-8a09-765794883524