Add #endif comments to gdb api header file

Author: danpoe

src/memory-analyzer/gdb_api.h

@@ -73,5 +73,5 @@ class gdb_inaccessible_memory_exceptiont : public cprover_exception_baset
 private:
   std::string reason;
 };
-#endif
-#endif
+#endif // CPROVER_MEMORY_ANALYZER_GDB_API_H
+#endif // __linux__