RIOT: Fix building RIOT with RIOT's utlist.h change

examples/riot/pkg_libcoap/Makefile

@@ -1,6 +1,6 @@
 PKG_NAME=libcoap
 PKG_URL=https://github.com/obgm/libcoap
-PKG_VERSION=f12b620cdecb00e7d4dcfb104f28f577c80f06cc
+PKG_VERSION=0c18ad522887ea70fd3588915e3ee04b633ab3ad
 PKG_LICENSE=BSD-2-Clause
 
 LIBCOAP_BUILD_DIR=$(BINDIR)/pkg/$(PKG_NAME)

include/coap3/coap_utlist_internal.h

@@ -21,6 +21,7 @@ NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
+#ifndef RIOT_VERSION
 #ifndef UTLIST_H
 #define UTLIST_H
 
@@ -1071,3 +1072,4 @@ do {
 #endif /* NO_DECLTYPE */
 
 #endif /* UTLIST_H */
+#endif /* ! RIOT_VERSION */