lex - yylval undeclared

Fix compilation with Ubuntu 16.04/yacc:
$ yacc -V
yacc - 1.9 20140715

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu Xenial Xerus (development branch)
Release:	16.04
Codename:	xenial

Change-Id: Ib1a6d7b7f9485db893465041ac6bce2a98ded2f7
Signed-off-by: Vincent JARDIN <vincent.jardin@6wind.com>
diff --git a/vppapigen/lex.c b/vppapigen/lex.c
index b9dc833..b41eb5e 100644
--- a/vppapigen/lex.c
+++ b/vppapigen/lex.c
@@ -54,6 +54,7 @@
 static int name_check (const char *, YYSTYPE *);
 static int name_compare (const char *, const char *);
 extern int yydebug;
+extern YYSTYPE yylval;
 
 unsigned int crc32c_table[256] = { 
   0x00000000, 0xF26B8303, 0xE13B70F7, 0x1350F3F4,