MINOR: Makefile: add MEMORY_POOLS to the list of DEBUG_xxx options

Add the DEBUG_MEMORY_POOLS option to the list of DEBUG_xxx options in
the Makefile.
diff --git a/Makefile b/Makefile
index 10c49fa..998a17d 100644
--- a/Makefile
+++ b/Makefile
@@ -232,7 +232,7 @@
 # DEBUG_MEM_STATS, DEBUG_DONT_SHARE_POOLS, DEBUG_FD,
 # DEBUG_NO_POOLS, DEBUG_FAIL_ALLOC, DEBUG_STRICT_NOCRASH, DEBUG_HPACK,
 # DEBUG_AUTH, DEBUG_SPOE, DEBUG_UAF, DEBUG_THREAD, DEBUG_STRICT, DEBUG_DEV,
-# DEBUG_TASK.
+# DEBUG_TASK, DEBUG_MEMORY_POOLS.
 DEBUG =
 
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