fixdep: remove unneeded memcpy() in parse_dep_file()

Each token in the depfile is copied to the temporary buffer 's' to
terminate the token with zero.  We do not need to do this any more
because the parsed buffer is now writable.  Insert '\0' directly in
the buffer without calling memcpy().

<limits.h> is no longer necessary. (It was needed for PATH_MAX).

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>