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

A string definition is inconsistent and thus causes a compilation error.
Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>

storage/tokudb/ha_tokudb.cc

@@ -379,7 +379,7 @@ static int free_share(TOKUDB_SHARE * share) {
     }
 
 const char *ha_tokudb::table_type() const {
-    extern const char * const tokudb_hton_name;
+    extern const char *tokudb_hton_name;
     return tokudb_hton_name;
 }