Avoid off_t in small_vector.h (#5936)

Fixes #5928 , on FreeBSD off_t is not defined in the headers we include.

Author: kripken

src/support/small_vector.h

@@ -185,7 +185,9 @@ template<typename T, size_t N> class SmallVector {
       return Iterator(*this) += off;
     }
 
-    off_t operator-(const Iterator& other) const { return index - other.index; }
+    difference_type operator-(const Iterator& other) const {
+      return index - other.index;
+    }
 
     bool operator==(const Iterator& other) const {
       return parent == other.parent && index == other.index;