package/libnss: Re-add ZLIB_INCLUDE_DIR varible

package/libnss: Re-add ZLIB_INCLUDE_DIR varible

The libnss patch adding ZLIB_INCLUDE_DIR is added to upstream on
2020-01-07 but the 3.49.1 release on 2020-01-13 does not contain this
patch, so we have actually prematurely removed it from Buildroot.

This only affects host-libnss when libzlib is not installed in the host
system. When building for the target, the toolchain-wrapper already
looks in the target sysroot default include path - where zlib.h is
installed.

Re-add this patch, so that we can build host-libnss 3.49.1.

Signed-off-by: Thomas Preston <thomas.preston@codethink.co.uk>
Reviewed-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>