updating headers from code review.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@9449 6f19259b-4bc3-4df7-8a09-765794883524

ShellPkg/Include/ShellBase.h

@@ -12,6 +12,9 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 
 **/
 
+#if !defined(__SHELL_BASE__)
+#define __SHELL_BASE__
+
 typedef enum {
 ///
 /// The operation completed successfully.
@@ -140,3 +143,4 @@ SHELL_SECURITY_VIOLATION    = 26,
 SHELL_CRC_ERROR             = 27
 }SHELL_STATUS;
 
+#endif //__SHELL_BASE__