Make Swap Hands use PROGMEM (#12284)

This converts the array that the Swap Hands feature uses to use PROGMEM,
and to read from that array, as such. Since this array never changes at
runtime, there is no reason to keep it in memory. Especially for AVR
boards, as memory is a precious resource.

keyboards/lets_split/lets_split.c

@@ -2,7 +2,7 @@
 
 #ifdef SWAP_HANDS_ENABLE
 __attribute__ ((weak))
-const keypos_t hand_swap_config[MATRIX_ROWS][MATRIX_COLS] = {
+const keypos_t PROGMEM hand_swap_config[MATRIX_ROWS][MATRIX_COLS] = {
 
   {{0, 4}, {1, 4}, {2, 4}, {3, 4}, {4, 4}, {5, 4}},
   {{0, 5}, {1, 5}, {2, 5}, {3, 5}, {4, 5}, {5, 5}},