patch 8.0.1304: CTRL-G/CTRL-T don't work with incsearch and empty pattern

Problem:    CTRL-G/CTRL-T don't work with incsearch and empty pattern.
Solution:   Use the last search pattern. (Christian Brabandt, closes #2292)
diff --git a/src/proto/search.pro b/src/proto/search.pro
index 41c2006..4e63bf3 100644
--- a/src/proto/search.pro
+++ b/src/proto/search.pro
@@ -7,6 +7,7 @@
 void restore_search_patterns(void);
 void save_last_search_pattern(void);
 void restore_last_search_pattern(void);
+char_u *last_search_pattern(void);
 void free_search_patterns(void);
 int ignorecase(char_u *pat);
 int ignorecase_opt(char_u *pat, int ic_in, int scs);