Specify minimum bison version 3.0+

Yosys works with bison 3.0 (or newer), but not bison 2.7 (the previous
release). Ideally, we would require "3" rather than "3.0" to give a
better error message, but bison 2.3, which still ships with macOS, does
not support major-only version requirements. With this change, building
with an outdated bison yields: `frontends/rtlil/rtlil_parser.y:25.10-14:
require bison 3.0, but have 2.3`.

frontends/rtlil/rtlil_parser.y

@@ -22,6 +22,8 @@
  *
  */
 
+%require "3.0"
+
 %{
 #include <list>
 #include "frontends/rtlil/rtlil_frontend.h"

frontends/verilog/verilog_parser.y

@@ -33,6 +33,8 @@
  *
  */
 
+%require "3.0"
+
 %{
 #include <list>
 #include <stack>