bc: zcommon_parse_expr() lost its flags argument

bc: zcommon_parse_expr() lost its flags argument

function                                             old     new   delta
zcommon_parse_expr                                     -      28     +28
zdc_program_execStr                                  474     472      -2
zbc_program_exec                                    4023    4021      -2
common_parse_expr                                     29       -     -29
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(add/remove: 1/1 grow/shrink: 0/2 up/down: 28/-33)             Total: -5 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>