blobmsg: fix array out of bounds GCC 10 warning

Fixes following warning reported by GCC 10.0.0 20191203:

 blobmsg.c:234:2: error: 'strcpy' offset 6 from the object at 'attr' is out of the bounds of referenced subobject 'name' with type 'uint8_t[0]' {aka 'unsigned char[0]'} at offset 6 [-Werror=array-bounds]
   234 |  strcpy((char *) hdr->name, (const char *)name);
       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 In file included from blobmsg.c:16:
 blobmsg.h:42:10: note: subobject 'name' declared here
    42 |  uint8_t name[];
       |          ^~~~

Reported-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>