systemc: Add the nonstandard sc_user exception type.

This type is not in the spec but is used in the tests.

Change-Id: I4537a33b0b5dcb3c72a091276d601c244c4a3862
Reviewed-on: https://gem5-review.googlesource.com/11284
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/ext/utils/sc_exception.hh

@@ -37,6 +37,14 @@ namespace sc_core
 
 typedef std::exception sc_exception;
 
+// Nonstandard
+class sc_user
+{
+  public:
+    sc_user() {}
+    sc_user(const sc_user &) {}
+};
+
 } // namespace sc_core
 
 #endif  //__SYSTEMC_EXT_UTIL_SC_EXCEPTION_HH__