various .h files: Add missing C++ extern wrappers

Updates #2405.

c/src/lib/libbsp/shared/umon/umon.h

@@ -61,4 +61,8 @@ int rtems_initialize_tfs_filesystem(
  */
 int umoncons_poll_read(int minor);
 
+#ifdef __cplusplus
+}
 #endif
+
+#endif /* __rtems_umon_h */

cpukit/posix/include/rtems/posix/ptimer.h

@@ -87,6 +87,9 @@ int timer_getoverrun(
   timer_t   timerid
 );
 
+#ifdef __cplusplus
+}
+#endif
 /** @} */
 
-#endif
+#endif /* _RTEMS_POSIX_PTIMER_H */

cpukit/rtems/include/rtems/rtems/dpmemimpl.h

@@ -20,6 +20,10 @@
 #include <rtems/rtems/dpmem.h>
 #include <rtems/score/objectimpl.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @defgroup ClassicDPMEMImpl Dual Ported Memory Manager Implementation
  *
@@ -104,5 +108,5 @@ RTEMS_INLINE_ROUTINE Dual_ported_memory_Control *_Dual_ported_memory_Get (
 }
 #endif
 
-#endif
+#endif /* _RTEMS_RTEMS_DPMEM_INL */
 /* end of include file */