Keyboard: Dactyl - Only initialize relevant variables (#3771)

This small change prevents an unused variable from being initialized, throwing a warning during compilation, thus ending fatally.

[07:47:10] [Daniels-MacBook-Pro-3] [dburkhol] [ /Users/dburkhol/git/qmk_firmware ] [develop ]
: make handwired/dactyl:default
QMK Firmware 0.6.97
Making handwired/dactyl with keymap default

avr-gcc (GCC) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Size before:
   text    data     bss     dec     hex filename
      0   19972       0   19972    4e04 .build/handwired_dactyl_default.hex

Compiling: keyboards/handwired/dactyl/twimaster.c                                                   [OK]
Compiling: keyboards/handwired/dactyl/matrix.c                                                     keyboards/handwired/dactyl/matrix.c:52:22: error: 'expander_col_pins' defined but not used [-Werror=unused-const-variable=]
 static const uint8_t expander_col_pins[MATRIX_COLS] = MATRIX_EXPANDER_COL_PINS;
                      ^~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
 [ERRORS]
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make[1]: *** [.build/obj_handwired_dactyl_default/matrix.o] Error 1
make: *** [handwired/dactyl:default] Error 1
Make finished with errors

0 files changed, 0 insertions, 0 deletions