README: add a missing necessary package

README: add a missing necessary package

src/dbtest.c requires 'gdbm-ndbm.h' or 'ndbm.h', both of which are
supplied by 'libgdbm-compat-dev' in the latest Ubuntu LTS. However,
this package is not a dependency of the currently listed packages.
Therefore, add it explicitly to the necessary packages list.

$ dpkg -L libgdbm-compat-dev
/.
/usr
/usr/include
/usr/include/dbm.h
/usr/include/gdbm-ndbm.h
/usr/include/ndbm.h

Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>