BaseTools/VfrCompile: Explicitly state format string for DebugMsg()

BaseTools/VfrCompile: Explicitly state format string for DebugMsg()

For calls to API DebugMsg(), explicitly state format string as "%s" when
the given variable list is a sting.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>