lib/krb5: prevent build failures of krb5_plugin_common on Windows

The Microsoft compiler cannot handle multiple const modifiers
for the same type.    It is also unhappy with the output pointer
parameter being declared const.

This change introduces new typedefs and cast of the dlsym() return
type to prevent warnings.

Change-Id: Ia92645efab8d2ec6745339a6f47c690782ae730a

lib/krb5/ccache_plugin.h

@@ -41,6 +41,6 @@ krb5_error_code KRB5_CALLCONV
 ccache_ops_plugin_load(krb5_context context,
 		       krb5_get_instance_func_t *func,
 		       size_t *n_ftables,
-		       const krb5_plugin_common_ftable *const **ftables);
+		       krb5_plugin_common_ftable_p **ftables);
 
 #endif /* HEIMDAL_KRB5_CCACHE_PLUGIN_H */