systemd: Fix busctl crash on aarch64 when setting output table format

systemd: Fix busctl crash on aarch64 when setting output table format

The enum used for column names is integer type while table_set_display() is parsing
arguments on size_t alignment which may result in assert in table_set_display() if
the size between types missmatch. This patch cast the enums to size_t.
An alternative solution would be to change the table_set_display() function
arguments to unsigned type.

Change-Id: I862db0168d58ecb3b7ee7dcf7f751be5a6121fd9