Fix implicit declaration warning

[yh-sb]

Fix the following implicit declaration warnings:

[1/4] Building C object CMakeFiles/rp2040.dir/third_party/pico-sdk/src/rp2_common/pico_flash/flash.c.obj
D:/dev/cpp/mcu-cpp/third_party/pico-sdk/src/rp2_common/pico_flash/flash.c: In function 'default_enter_safe_zone_timeout_ms':
D:/dev/cpp/mcu-cpp/third_party/pico-sdk/src/rp2_common/pico_flash/flash.c:155:33: warning: implicit declaration of function 'make_timeout_time_ms' [-Wimplicit-function-declaration]
  155 |         absolute_time_t until = make_timeout_time_ms(timeout_ms);
      |                                 ^~~~~~~~~~~~~~~~~~~~
D:/dev/cpp/mcu-cpp/third_party/pico-sdk/src/rp2_common/pico_flash/flash.c:156:77: warning: implicit declaration of function 'time_reached' [-Wimplicit-function-declaration]
  156 |         while (lockout_state[core_num] != FREERTOS_LOCKOUT_LOCKEE_READY && !time_reached(until)) {
      |                                                                             ^~~~~~~~~~~~