preproc: don't pass an enum to %s

Need to use pp_directives[directive], not the plain directive, which
is an enum.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

preproc.c

@@ -2023,7 +2023,8 @@ static size_t nasm_unquote_cstr(char *qstr, enum preproc_token directive)
     size_t clen = strlen(qstr);
 
     if (len != clen)
-	error(ERR_NONFATAL, "NUL character in `%s' directive", directive);
+	error(ERR_NONFATAL, "NUL character in `%s' directive",
+	      pp_directives[directive]);
 
     return clen;
 }