patch 9.0.0771: cannot always tell the difference beween tex and rexx files

Problem:    Cannot always tell the difference beween tex and rexx files.
Solution:   Recognize tex by a leading backslash. (Martin Tournoij,
            closes #11380)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    771,
 /**/
     770,
 /**/