mesh: Enforce proper compilation.

The current method relies heavily on the linker/compiler to
do the correct operation. Which is to eliminate the code that will
never get called. This posses a problem if the build even changes
by a smallest fraction.
The current patch will enforce proper inclusion of the code at the
pre-processing stage. Thereby not relying on the compiler/linker to
do the right thing.

nimble/host/mesh/src/prov.c

@@ -1095,7 +1095,7 @@ static void prov_data(const u8_t *data)
 	/* After PB-GATT provisioning we should start advertising
 	 * using Node Identity.
 	 */
-	if (identity_enable) {
+	if (IS_ENABLED(CONFIG_BT_MESH_GATT_PROXY) && identity_enable) {
 		bt_mesh_proxy_identity_enable();
 	}