Cast result of i64abs() back to int64

Without the cast, the return type could be long or long long,
depending on what int64 is underneath.  This doesn't affect code
correctness, but it could result in format-mismatch warnings when
attempting to printf such values using PRId64.

Reported-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+hUKGJc4s+Wyb3EFOQNN9VVK+Qv40r2LK41o9PkS9ThxviTvQ@mail.gmail.com

src/include/c.h

@@ -1278,9 +1278,9 @@ extern int	fdatasync(int fildes);
  * Similarly, wrappers around labs()/llabs() matching our int64.
  */
 #if SIZEOF_LONG == 8
-#define i64abs(i) labs(i)
+#define i64abs(i) ((int64) labs(i))
 #elif SIZEOF_LONG_LONG == 8
-#define i64abs(i) llabs(i)
+#define i64abs(i) ((int64) llabs(i))
 #else
 #error "cannot find integer type of the same size as int64_t"
 #endif