TLS_TCB_AT_TP: avoid warning about undef cpp token

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libpthread/nptl_db/structs.def

@@ -77,7 +77,7 @@ DB_STRUCT_ARRAY_FIELD (dtv, dtv)
 # define pointer_val pointer.val /* Field of anonymous struct in dtv_t.  */
 DB_STRUCT_FIELD (dtv_t, pointer_val)
 #endif
-#if !defined IS_IN_libpthread || TLS_TCB_AT_TP
+#if !defined IS_IN_libpthread || (defined TLS_TCB_AT_TP && TLS_TCB_AT_TP)
 DB_STRUCT_FIELD (pthread, dtvp)
 #endif