fixes a regression where memset was used without including <string.h>

tests/reject/twrongiter.nim

@@ -0,0 +1,13 @@
+discard """
+line: 14
+errormsg: "type mismatch"
+"""
+
+proc first(it: iterator(): int): seq[int] =
+  return @[]
+
+iterator primes(): int =
+  yield 1
+
+for i in first(primes):
+  break