patch 9.0.1583: get E304 when using 'cryptmethod' "xchacha20v2"

Problem:    Get E304 when using 'cryptmethod' "xchacha20v2". (Steve Mynott)
Solution:   Add 4th crypt method to block zero ID check.  Avoid syncing a swap
            file before reading the file. (closes #12433)
diff --git a/src/proto/crypt.pro b/src/proto/crypt.pro
index c067829..d599238 100644
--- a/src/proto/crypt.pro
+++ b/src/proto/crypt.pro
@@ -22,6 +22,7 @@
 void crypt_decode_inplace(cryptstate_T *state, char_u *buf, size_t len, int last);
 void crypt_free_key(char_u *key);
 void crypt_check_method(int method);
+void crypt_check_swapfile_curbuf(void);
 void crypt_check_current_method(void);
 char_u *crypt_get_key(int store, int twice);
 void crypt_append_msg(buf_T *buf);