libctf: fix the type of ctf_enum.cte_value

This stops the file format from depending on the size of the host int.
(It does mean that we cannot encode enums with a value > 2^32 on
platforms with an int > 2^32: this will be fixed in the next format
revision.)

include/
	* ctf.h (ctf_enum.cte_value): Fix type to int32_t.