package/libsoxr: cleanup configure options

BUILD_SHARED_LIBS is already driven by the cmake-package
infrastructure.

[Thomas: amend to really remove the BUILD_SHARED_LIBS option from the
package, after checking on IRC with Samuel.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>