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/search.c b/src/search.c
index d2554e2..850fd06 100644
--- a/src/search.c
+++ b/src/search.c
@@ -567,7 +567,9 @@
 	    saved_spats[idx].pat = NULL;
 	else
 	    saved_spats[idx].pat = vim_strsave(spats[idx].pat);
+# ifdef FEAT_SEARCH_EXTRA
 	saved_spats_last_idx = last_idx;
+# endif
     }
 # ifdef FEAT_SEARCH_EXTRA
     /* If 'hlsearch' set and search pat changed: need redraw. */