libnewt: fix recompile error

libnewt: fix recompile error

Fixed:
NOTE: make -j 32
make: *** No rule to make target `/path/to/sysroot/4.9.0/include/stddef.h', needed by `test.o'.  Stop.

This happens when upgrade gcc from 4.9.0 to 4.9.1, and the .depend isn't
regenerated when recompile, the content of the .depend are:

[snip]
test.o: /path/to/sysroot/4.9.0/include/stddef.h
[snip]

And Makefile includes the .depend file if it exists, so there would be
errors when /path/to/sysroot/4.9.0/include/stddef.h doesn't exist.

Remove .depend will fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>