tools/liblockdep: Use realpath for srctree and objtree

If BUILD_SRC or CURDIR contains tailing '/', the file names passed to gcc will
contain '//'. It will be contained .o's in debuginfo, then confuse debugedit:

https://bugzilla.redhat.com/show_bug.cgi?id=304121



This patch uses realpath command to makesure potential tailing '/'s are removed.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Geng Hui <hui.geng@huawei.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>