Fix build with Visual Studio 2013

Add #include <cstdint> to src/utils/context_free_grammar_generator.hpp. It
appears that without it, uint32_t ends up being different types in the
header and the implementation file (at least in Visual Studio 2013), which
makes the compiler complain that the print_nonterminal() function hasn't
been declared.

src/utils/context_free_grammar_generator.hpp

@@ -21,6 +21,7 @@
 #include <map>
 #include <list>
 #include <vector>
+#include <cstdint>
 
 class context_free_grammar_generator : public name_generator
 {