check-symfile: Use pythonesque string formatting instead of perl

Commit d30a1ad044 translated the symbol file checker from perl to
python by doing a literal translation in most cases. Unfortunately one
string formatting operation was not really translated into python
leaving users with non-helpful error:

'Symbol $1 is listed twice'

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

scripts/check-symfile.py

@@ -52,7 +52,7 @@ with open(symfile, "r") as fh:
         line = line.strip(";")
 
         if line in wantsyms:
-            print("Symbol $1 is listed twice", file=sys.stderr)
+            print("Symbol %s is listed twice" % line, file=sys.stderr)
             ret = 1
         else:
             wantsyms[line] = True