input/joystick: magellan: Mark __nonstring look-up table const

GCC 15's new -Wunterminated-string-initialization notices that the
16 character lookup table "nibbles" (which is not used as a C-String)
needs to be marked as "nonstring":

drivers/input/joystick/magellan.c: In function 'magellan_crunch_nibbles':
drivers/input/joystick/magellan.c:51:44: warning: initializer-string for array of 'unsigned char' truncates NUL terminator but destination lacks 'nonstring' attribute (17 chars into 16 available) [-Wunterminated-string-initialization]
   51 |         static unsigned char nibbles[16] = "0AB3D56GH9:K<MN?";
      |                                            ^~~~~~~~~~~~~~~~~~

While the annotation was added in commit 48709ee1fa9a ("gcc-15: add
'__nonstring' markers to byte arrays") also mark the table as const.

Signed-off-by: Kees Cook <kees@kernel.org>