Add 'extern C' if simulator is written in C++

sim/common/Changelog:
2018-12-31  Pavel I. Kryukov  <kryukov@frtk.ru>

        * sim-base.h: Add 'extern C' if header is compiled with C++

sim/common/sim-base.h

@@ -55,6 +55,10 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #ifndef SIM_BASE_H
 #define SIM_BASE_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* Pre-declare certain types. */
 
 /* typedef <target-dependant> sim_cia; */
@@ -224,4 +228,8 @@ typedef struct {
 SIM_DESC sim_state_alloc (SIM_OPEN_KIND kind, host_callback *callback);
 void sim_state_free (SIM_DESC);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* SIM_BASE_H */