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>

mbim-type.h

@@ -18,7 +18,7 @@
 struct mbim_string {
 	uint32_t offset;
 	uint32_t length;
-};
+} __attribute__((packed));
 
 struct mbim_enum {
 	uint32_t key;