Change signature of kgets_func2() to be more like hgetln()

In particular the second parameter should be size_t, not int.
The incorrect version worked when using hgetln(), mainly because
the ABI passes arguments in registers.  This ensures it will
work because the types match instead of due to a happy accident.

Hat-tip to John Marshall for spotting this.

htslib/kstring.h

@@ -130,7 +130,8 @@ extern "C" {
     HTSLIB_EXPORT
 	int kgetline(kstring_t *s, kgets_func *fgets, void *fp);
 
-	typedef ssize_t kgets_func2(char *, int, void *);
+    // This matches the signature of hgetln(), apart from the last pointer
+	typedef ssize_t kgets_func2(char *, size_t, void *);
     HTSLIB_EXPORT
 	int kgetline2(kstring_t *s, kgets_func2 *fgets, void *fp);