umbim: fix compilation with GCC 10
When building umbim with GCC 10, a bunch of these errors appear:
cli.c: In function 'mbim_device_caps_response':
cli.c:55:29: error: taking address of packed member of 'struct mbim_basic_connect_device_caps_r' may result in an unaligned pointer value [-Werror=address-of-packed-member]
55 | deviceid = mbim_get_string(&caps->deviceid, buffer);
| ^~~~~~~~~~~~~~~
Fix them by disabling padding between fields of struct mbim_string.
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>