remove the first '/' in path

support we have the following directory structure:

root/
    a/
        a.cc
    b/
        b.h

and in a.cc, we include b.h

    #include "b.h"

and in the original implementation, it would be changed to

    #include "/b/b.h"

this is because `${hdr_path#$root}` operation is incomplete for this scenario:

        hdr_path = "/root/b/b.h"
        root = /root"