bind: Fix parallel make issue

bind: Fix parallel make issue

Fixed:
unix/os.o: file not recognized: File truncated
collect2: error: ld returned 1 exit status

This is because os.o was built twice:
* The implicity rule (depends on unix/os.o)
* The "make all" in unix subdir (depends on unix/os.o)

Depend on subdirs which is unix only rather than unix/os.o will fix the
problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>