cgo: include all enums in the CGo Go AST

Not all enums may be used as a type anywhere, which was previously the
only way to include an enum in the AST. This commit makes sure all enums
are included.

cgo/testdata/types.go

@@ -38,6 +38,9 @@ typedef enum option {
 	optionF,
 	optionG,
 } option_t;
+enum unused {
+	unused1 = 5,
+};
 
 // Anonymous enum.
 typedef enum {

cgo/testdata/types.out.go

@@ -12,6 +12,7 @@ const C.optionD = -4
 const C.optionE = 10
 const C.optionF = 11
 const C.optionG = 12
+const C.unused1 = 5
 
 type C.int16_t = int16
 type C.int32_t = int32
@@ -81,3 +82,4 @@ type C.struct_type1 struct {
 }
 type C.struct_type2 struct{ _type C.int }
 type C.enum_option C.int
+type C.enum_unused C.uint