Fix s/ITERTOR/ITERATOR/ typo in simplehash.h.

This could lead to problem when simplehash.h is used to define two
different types of hashtable visible in the same translation unit.

Reported-By: Josh Soref
Discussion: https://postgr.es/m/CACZqfqCC7WdBAY=rQePb9-qW1rjdaTdHsV5KoVejHkDb6qrtOg@mail.gmail.com

src/include/lib/simplehash.h

@@ -885,7 +885,7 @@ SH_STAT(SH_TYPE *tb)
 #undef SH_STATUS
 #undef SH_STATUS_EMPTY
 #undef SH_STATUS_IN_USE
-#undef SH_ITERTOR
+#undef SH_ITERATOR
 
 /* external function names */
 #undef SH_CREATE