keypad: Fix compilation error for clang

Function hdy_keypad_button_get_digit is declared to return 'char' in
src/hdy-keypad-button-private.h but defined to return 'const char' in
src/hdy-keypad-button.c. This is not allowed by clang. Since it is
unusual to mark a return value itself as const, just drop const here.
8 jobs for wip/lantw/keypad-Fix-compilation-error-for-clang in 12 minutes and 9 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build-debian-clang #232951
librem5

00:03:41

passed build-debian-gcc #232949
librem5

00:02:52

passed build-debian-gcc-static #232950
librem5

00:02:48

passed build-fedora-gcc #232956
librem5

00:02:05

failed build-fedora-gcc #232952
librem5

00:03:20

 
  Docs
passed build-gtkdoc #232953
librem5

00:01:39

 
  Test
passed unit-test #232954
librem5

00:02:01

44.1%
 
  Package
passed package-debian #232955
librem5

00:02:39