transport: move the cert type enum to types.h

This should make the mingw compiler happy.

include/git2/transport.h

@@ -20,22 +20,6 @@
  */
 GIT_BEGIN_DECL
 
-/**
- * Type of host certificate structure that is passed to the check callback
- */
-typedef enum git_cert_t {
-        /**
-         * The `data` argument to the callback will be a pointer to
-         * the DER-encoded data.
-         */
-	GIT_CERT_X509,
-        /**
-         * The `data` argument to the callback will be a pointer to a
-         * `git_cert_hostkey` structure.
-         */
-	GIT_CERT_HOSTKEY_LIBSSH2,
-} git_cert_t;
-
 /**
  * Hostkey information taken from libssh2
  */