kconfig: Make expr_copy() take a const argument

Fixes
scripts/kconfig/expr.c: In function ‘expr_get_leftmost_symbol’:
scripts/kconfig/expr.c:1026:2: warning: passing argument 1 of ‘expr_copy’ discards qualifiers from pointer target type
scripts/kconfig/expr.c:67:14: note: expected ‘struct expr *’ but argument is of type ‘const struct expr *’

Signed-off-by: Michal Marek <mmarek@suse.cz>