blobmsg: work around false positive gcc -Warray-bounds warnings

blobmsg: work around false positive gcc -Warray-bounds warnings

Using the return value of blobmsg_name as input argument to strcpy can lead
to warnings like these:

error: 'strcpy' offset 6 from the object at 'cur' is out of the bounds of referenced subobject 'name' with type 'uint8_t[]' {aka 'unsigned char[]'} at offset 6 [-Werror=array-bounds]

Fix this by replacing hdr->name with the equivalent hdr + 1

Signed-off-by: Felix Fietkau <nbd@nbd.name>