package/unixodbc: add optional dependency to libtool

package/unixodbc: add optional dependency to libtool

To get reproducible builds of unixodbc add libtool as optional
dependency.

Without libtool:

$ host/usr/bin/x86_64-linux-readelf -a target/usr/lib/libodbc.so.2.0.0 | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

With libtool:

$ host/usr/bin/x86_64-linux-readelf -a target/usr/lib/libodbc.so.2.0.0 | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libltdl.so.7]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

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