x11-wm/tvtwm: Fix build with LLVM 11

The fix for this is identical to that of x11-wm/piewm; remove the patch that
explicitly de-externs yylineno when it's explicitly defined in another
compilation unit (by flex).

PR:		248976
MFH:		2020Q3 (-fno-common build fix)

x11-wm/tvtwm/files/patch-gram.y

@@ -1,11 +0,0 @@
---- gram.y.orig	1993-11-22 20:10:49 UTC
-+++ gram.y
-@@ -79,7 +79,7 @@ unsigned int mods_used = (ShiftMask | Co
- extern int do_single_keyword(), do_string_keyword(), do_number_keyword();
- extern name_list **do_colorlist_keyword();
- extern int do_color_keyword(), do_string_savecolor();
--extern int yylineno;
-+int yylineno;
- %}
- 
- %union