sys/riotboot/slot: fix failed build

When riotboot/slot.h is included, but riotboot is not enabled in the
build, a compiler error occures. This is because SLOT0_LEN is not
defined. This patch fixes this by surrounding the offending macrro with
a conditional compile. `riotboot_slot_size()` may still be called, in
which case it returns 0 for the size of any slot given.

sys/include/riotboot/slot.h

@@ -116,8 +116,10 @@ void riotboot_slot_dump_addrs(void);
 static inline size_t riotboot_slot_size(unsigned slot)
 {
     switch (slot) {
+#if NUM_SLOTS >= 1
     case 0:
         return SLOT0_LEN;
+#endif
 #if NUM_SLOTS == 2
     case 1:
         return SLOT1_LEN;