apr: requires shared library support

The apr library requires shared library support unconditionally, so
make it depend on !BR2_PREFER_STATIC_LIB, so that architectures that
support static libraries only do not make this package available.

Solves
http://autobuild.buildroot.org/results/f4cd9751e85e9ee7fde2e9479e0f01ab2be93e84/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>