service.c: fix build on gcc 10

service.c: fix build on gcc 10

Resolve error:
/Users/kevin/wrt/build_dir/target-x86_64_musl/umdns-2020-06-08-d13290b4/service.c: In function 'service_load_blob':
/Users/kevin/wrt/build_dir/target-x86_64_musl/umdns-2020-06-08-d13290b4/service.c:240:10: error: 'strcpy' offset 6 from the object at 'b' is out of the bounds of referenced subobject 'name' with type 'uint8_t[]' {aka 'unsigned char[]'} at offset 6 [-Werror=array-bounds]
  240 |  s->id = strcpy(d_id, blobmsg_name(b));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /Users/kevin/wrt/staging_dir/target-x86_64_musl/usr/include/libubus.h:23,
                 from /Users/kevin/wrt/build_dir/target-x86_64_musl/umdns-2020-06-08-d13290b4/service.c:23:
/Users/kevin/wrt/staging_dir/target-x86_64_musl/usr/include/libubox/blobmsg.h:42:10: note: subobject 'name' declared here
   42 |  uint8_t name[];
      |          ^~~~
cc1: all warnings being treated as errors
make[5]: *** [CMakeFiles/umdns.dir/build.make:132: CMakeFiles/umdns.dir/service.c.o] Error 1

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>