maint: avoid discarded-qualifiers warnings with SELinux 3.5

maint: avoid discarded-qualifiers warnings with SELinux 3.5

Since SELinux version 3.5, the return value of context_str(3) is
declared as const; see:
  https://github.com/SELinuxProject/selinux/commit/dd98fa322766
Therefore, GCC complains (here with -Werror):
  src/selinux.c: In function 'defaultcon':
  src/selinux.c:152:16: error: assignment discards 'const' qualifier \
    from pointer target type [-Werror=discarded-qualifiers]
    152 |   if (!(constr = context_str (tcontext)))
        |                ^
  src/selinux.c: In function 'restorecon_private':
  src/selinux.c:252:16: error: assignment discards 'const' qualifier \
    from pointer target type [-Werror=discarded-qualifiers]
    252 |   if (!(constr = context_str (tcontext)))
      |                ^

* src/selinux.c (defaultcon): Define CONSTR as const.
(restorecon_private): Likewise.