rbcfg: Add missing mode to open call

rbcfg: Add missing mode to open call

When open() is called with O_CREAT a 3. parameter has to be given with
the file system permissions of the new file.

Not giving this is an error, which results in a compile error with glibc.

This fixes the following compile error with glibc:
----------------------------------------------------------------------
In file included from /include/fcntl.h:329,
                 from main.c:18:
In function 'open',
    inlined from 'rbcfg_update' at main.c:501:7:
/include/bits/fcntl2.h:50:4: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
    __open_missing_mode ();
    ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>