opt: Fix -Wmissing-field-initializers warning

opt: Fix -Wmissing-field-initializers warning

OPT_ENDTABLE does not initalise all the elements in stuct opt_table.
When compliling with -Wextra -Wmissing-field-initializers is enabled,
which produces the following warning:

        iviewiir.c:299:9: error: missing field 'cb' initializer
              [-Wmissing-field-initializers]
                OPT_ENDTABLE
                ^
        In file included from iviewiir.c:12:
        ./ccan/opt/opt.h:82:38: note: instantiated from:
        #define OPT_ENDTABLE { NULL, OPT_END }
                                             ^
By changing the definition of OPT_ENDTABLE to initalise all 7 elements
of struct opt_table, the warning is silenced.