lib/bootconfig: fix signed comparison in xbc_node_get_data()

lib/bootconfig: fix signed comparison in xbc_node_get_data()

  lib/bootconfig.c:188:28: warning: comparison of integer expressions
  of different signedness: 'int' and 'size_t' [-Wsign-compare]

The local variable 'offset' is declared as int, but xbc_data_size is
size_t.  Change the type to size_t to match and eliminate the warning.

Link: https://lore.kernel.org/all/20260318155919.78168-10-objecting@objecting.org/
Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>