MDEV-7526: TokuDB doesn't build on OS X

This patch fixes one compilation error related to __db_lsn struct. The
struct can not be defined as empty according to the main C standard.
In C++, this is handled by forcing a size of 1. To eliminate the error
we add a dummy char field of size 1. This has no effect on the C++
compiled code, but also removes the compiler error.

storage/tokudb/ft-index/buildheader/make_tdb.cc

@@ -481,7 +481,9 @@ static void print_db_key_range_struct (void) {
 
 static void print_db_lsn_struct (void) {
     field_counter=0;
-    sort_and_dump_fields("db_lsn", false, NULL);
+    /* A dummy field to make sizeof(DB_LSN) equal in C and C++ */
+    const char *extra[] = { "char dummy", NULL };
+    sort_and_dump_fields("db_lsn", false, extra);
 }
 
 static void print_dbt_struct (void) {