gxtags: fix use of ##locat-position (#1082)

As of

gambit/gambit@556a884
global ##locat-start-position should be used instead of
##locat-position.

src/tools/gxtags.ss

@@ -261,7 +261,7 @@
 
 (def (source-location-line locat)
   (if (##locat? locat)
-    (let (filepos (##position->filepos (##locat-position locat)))
+    (let (filepos (##position->filepos (##locat-start-position locat)))
       (fx1+ (##filepos-line filepos)))
     1))