bc: zcommon_parse_expr() has only one user, fold it in

bc: zcommon_parse_expr() has only one user, fold it in

function                                             old     new   delta
zbc_program_exec                                    4021    4046     +25
zdc_parse_expr                                       589     586      -3
zcommon_parse_expr                                    28       -     -28
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/1 up/down: 25/-31)             Total: -6 bytes

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