util: make set's deleted_key_value declaration consistent with hash table one

util: make set's deleted_key_value declaration consistent with hash table one

This also silences following clang warnings:
no previous extern declaration for non-static variable 'deleted_key' [-Werror,-Wmissing-variable-declarations]
const void *deleted_key = &deleted_key_value;
            ^
no previous extern declaration for non-static variable 'deleted_key_value'
      [-Werror,-Wmissing-variable-declarations]
uint32_t deleted_key_value;
         ^

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>