lib/bootconfig: use size_t for strlen result in xbc_node_match_prefix()

lib/bootconfig: use size_t for strlen result in xbc_node_match_prefix()

  lib/bootconfig.c:198:19: warning: conversion from 'size_t' to 'int'
  may change value [-Wconversion]
  lib/bootconfig.c:200:33: warning: conversion to '__kernel_size_t'
  from 'int' may change the sign of the result [-Wsign-conversion]

strlen() returns size_t but the result was stored in an int.  The value
is then passed back to strncmp() which expects size_t, causing a second
sign-conversion warning on the round-trip.  Use size_t throughout to
match the API types.

Link: https://lore.kernel.org/all/20260318155919.78168-11-objecting@objecting.org/
Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>