package/libiio: fix install of C# bindings

package/libiio: fix install of C# bindings

libiio-sharp.dll.mdb is not installed since version 0.19.0 and
https://github.com/analogdevicesinc/libiio/commit/8b571969fa245fb25c6431340f23ff157e34841b

Indeed, EXISTS "${LIBIIO_CS_DLL}.mdb" will always return false because
the file will not exist when cmake will create the makefiles.

Fixes:
 - http://autobuild.buildroot.org/results/42ffbd07d0e998c75d1afde6a5db7d6418c10f45

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>