package/libsigrok: fix libftdi support

package/libsigrok: fix libftdi support

libsigrok optionally depends on libftdi1, not libftdi. This was already
the case for a long time, but until the recent commit 01b30e5d69f, all
the configure options were wrong so it would use the automatic check for
availability of libftdi1. Now we pass the --with-libftdi option
explicitly, configure will fail if libftdi1 is not available.

Fixes:
 - http://autobuild.buildroot.net/results/ec1f9f57944139b24738c1be529c4fc4b128a516

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>