patch 8.1.2005: the regexp.c file is too big

Problem:    The regexp.c file is too big.
Solution:   Move the backtracking engine to a separate file. (Yegappan
            Lakshmanan, closes #4905)

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2005,
 /**/
     2004,
 /**/