MdeModulePkg/RegularExpressionDxe: Fix XCODE5 build failure

https://bugzilla.tianocore.org/show_bug.cgi?id=572

The ErrorMessage local variable in OnigurumaMatch() should
be type OnigUChar instead of type CHAR8.  This resolves
a build failure with the XCODE5 tool chain.

Cc: Andrew Fish <afish@apple.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Andrew Fish <afish@apple.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>