MINOR: stick-tables: Make static_table_key a struct variable instead of a pointer

MINOR: stick-tables: Make static_table_key a struct variable instead of a pointer

First, this variable does not need to be publicly exposed because it is only
used by stick_table functions. So we declare it as a global static in
stick_table.c file. Then, it is useless to use a pointer. Using a plain struct
variable avoids any dynamic allocation.