patch 8.1.0991: cannot build with a mix of features

Problem:    Cannot build with FEAT_EVAL defined and FEAT_SEARCH_EXTRA
            undefined, and with FEAT_DIFF defined and FEAT_EVAL undefined.
Solution:   Add a couple of #ifdefs. (closes #4067)
diff --git a/src/version.c b/src/version.c
index f9fea1c..1824903 100644
--- a/src/version.c
+++ b/src/version.c
@@ -780,6 +780,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    991,
+/**/
     990,
 /**/
     989,