lib/bootconfig: use size_t for key length tracking in xbc_verify_tree()

lib/bootconfig.c:839:24: warning: conversion from 'size_t' to 'int'
may change value [-Wconversion]
lib/bootconfig.c:860:32: warning: conversion from 'size_t' to 'int'
may change value [-Wconversion]
lib/bootconfig.c:860:29: warning: conversion to 'size_t' from 'int'
may change the sign of the result [-Wsign-conversion]

The key length variables len and wlen accumulate strlen() results but
were declared as int, causing truncation and sign-conversion warnings.
Change both to size_t to match the strlen() return type and avoid
mixed-sign arithmetic.

Link: https://lore.kernel.org/all/20260318155919.78168-13-objecting@objecting.org/

Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>