snapper: fix buildpaths QA warning in snbk binary

configure.ac resolves SHA256SUM_BIN and LS_BIN via AC_PATH_PROG, which
searches $PATH at configure time. Since hosttools is early in $PATH
during the build, these resolved to the absolute
build/tmp/hosttools/{sha256sum,ls} paths and got baked into config.h
via AC_DEFINE_UNQUOTED, ending up as string literals in the compiled
snbk binary:

  WARNING: snapper-0.13.1-r0 do_package_qa: QA Issue: File
  /usr/sbin/snbk in package snapper contains reference to TMPDIR
  [buildpaths]

The recipe already has a "Avoid HOSTTOOLS path in binaries" block that
overrides several of these AC_PATH_PROG variables (DIFF_BIN, RM_BIN,
etc.) with relocatable ${bindir} paths so autoconf skips the $PATH
search, but it was missing SHA256SUM_BIN and LS_BIN, which snbk (a
newer client tool) depends on. Add the same override for both.

Verified via cleansstate rebuild: log.do_package_qa no longer contains
any buildpaths warnings, and `strings` on the rebuilt snbk binary no
longer contains any build/tmp/hosttools paths.

AI-Generated: Uses Claude Code
Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-oe/recipes-support/snapper/snapper_0.13.1.bb

@@ -26,6 +26,8 @@ export MKDIR_BIN = "${bindir}/mkdir"
 export TOUCH_BIN = "${bindir}/touch"
 export CP_BIN = "${bindir}/cp"
 export REALPATH_BIN = "${bindir}/realpath"
+export SHA256SUM_BIN = "${bindir}/sha256sum"
+export LS_BIN = "${bindir}/ls"
 
 do_install:append() {
 	install -d ${D}${sysconfdir}/sysconfig