Create a macro for module declarations

The modules are now declared with a common macro. This allows future
additions to the module loading process while also making the loaded
symbol name a constant.

server/core/load_utils.c

@@ -194,7 +194,7 @@ void *load_module(const char *module, const char *type)
             return NULL;
         }
 
-        void *sym = dlsym(dlhandle, "GetModuleObject");
+        void *sym = dlsym(dlhandle, MXS_MODULE_SYMBOL_NAME);
 
         if (sym == NULL)
         {