Un-constify pointer_valuet data

So that we can assume the implicit assignment operator.

Author: petr-bauch

src/memory-analyzer/gdb_api.h

@@ -81,10 +81,10 @@ class gdb_apit
     {
     }
 
-    const memory_addresst address;
-    const std::string pointee;
-    const std::string character;
-    const optionalt<std::string> string;
+    memory_addresst address;
+    std::string pointee;
+    std::string character;
+    optionalt<std::string> string;
     bool valid;
   };