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)

src/if_ruby.c

@@ -506,7 +506,11 @@ SIGNED_VALUE rb_num2long_stub(VALUE x)
 {
     return dll_rb_num2long(x);
 }
+#  if defined(DYNAMIC_RUBY_VER) && DYNAMIC_RUBY_VER >= 26
+VALUE rb_int2big_stub(intptr_t x)
+#  else
 VALUE rb_int2big_stub(SIGNED_VALUE x)
+#  endif
 {
     return dll_rb_int2big(x);
 }

src/version.c

@@ -783,6 +783,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    871,
 /**/
     870,
 /**/