ShellPkg: Fix several GCC compiler warnings

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10890 6f19259b-4bc3-4df7-8a09-765794883524

ShellPkg/Library/UefiShellLevel2CommandsLib/Cp.c

@@ -111,6 +111,8 @@ CopySingleFile(
           *Resp = Response;
         case ShellPromptResponseYes:
           break;
+        default:
+          return SHELL_ABORTED;
       }
     }
   }