Fix build failure in darwin-nat.c

Fix build failure in darwin-nat.c

Fix:

/Users/simark/src/binutils-gdb/gdb/darwin-nat.c:2404:3: error: no matching function for call to 'add_setshow_boolean_cmd'
  add_setshow_boolean_cmd ("mach-exceptions", class_support,
  ^~~~~~~~~~~~~~~~~~~~~~~

gdb/ChangeLog:

	* darwin-nat.c (set_enable_mach_exceptions): Constify parameter.