Since version 14.1 it seems ExecGetJunkAttribute is defined in the server/executor/executor.h
header file as inline function and the symbol is no longer exported from the postgres binary.
So to be able to load mysql_fdw.so we need to include that include file when compiling.
diff -wpruN '--exclude=*.orig' a~/mysql_fdw.c a/mysql_fdw.c
--- a~/mysql_fdw.c	1970-01-01 00:00:00
+++ a/mysql_fdw.c	1970-01-01 00:00:00
@@ -35,6 +35,9 @@
 #include "commands/defrem.h"
 #include "commands/explain.h"
 #include "catalog/heap.h"
+#if PG_VERSION_NUM >= 140000
+#include "executor/executor.h"
+#endif
 #include "foreign/fdwapi.h"
 #include "miscadmin.h"
 #include "mysql_query.h"