sane-backends: disable static configuration

sane-backends: disable static configuration

While sane-backends can be compiled statically, it requires
selection of a subset of scanner backends. Without this, all backends
are compiled and link errors such as the following are generated:

../backend/.libs/libsane.a(libepsonds_la-epsonds.o):(.bss+0xc): multiple definition of `source_list'
../backend/.libs/libsane.a(libepson2_la-epson2.o):(.data+0x40): first defined here
../backend/.libs/libsane.a(libepsonds_la-epsonds.o):(.data+0x10): multiple definition of `mode_params'
../backend/.libs/libsane.a(libepson2_la-epson2.o):(.data+0x0): first defined here
collect2: error: ld returned 1 exit status

Buildroot currently doesn't allow fine grained selection of backends, so
this disables static builds.

Fixes:

http://autobuild.buildroot.net/results/89c/89c6e4ec1596def74e72eb071ef4bf0f1a7cc9f8/
http://autobuild.buildroot.net/results/92a/92a54c99a9ba918ece4c56d7beb985729bee700b/
http://autobuild.buildroot.net/results/1b9/1b9eac5303dfed638063f3b18dfff5571a2c4231/
http://autobuild.buildroot.net/results/366/366979d0bd21dc2bf7f0336b9653ef3fd4e11123/

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>