patch 8.1.0871: build error when building with Ruby 2.6.0

Problem:    Build error when building with Ruby 2.6.0.
Solution:   Change argument of rb_int2big_stub(). (Android Baumann,
            closes #3899)
diff --git a/src/version.c b/src/version.c
index 3e0dc7e..3d24067 100644
--- a/src/version.c
+++ b/src/version.c
@@ -784,6 +784,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    871,
+/**/
     870,
 /**/
     869,