package/openipmi: add optional support for gdbm

When gdbm was compiled before, openipmi will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/ipmi_sim | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libgdbm.so.4]
[...]

The build system offers no option to en-/disable gdbm support,
check configure.ac, line 309+

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>