Teach groveler cstruct :count :auto to work on arrays anywhere in the struct.

The :count :auto logic assumed the struct member was an array which
filled the remainder of the struct, and would generate incorrect cffi
cstruct forms when any other struct members followed one declared
:count :auto.

The following C struct definition and groveler cstruct form
demonstrate this:

struct charbuf {
	char three_chars[3];
	int clobbered_int
};

(cstruct charbuf "struct charbuf"
  (three-chars "three_chars" :type :char :count :auto)
  (clobbered-int "clobbered_int" :type :int))