c-icap: fix incorrect dependency on BR2_PREFER_STATIC_LIB

BR2_PREFER_STATIC_LIB has been changed/renamed quite some time ago to
BR2_STATIC_LIBS, but the c-icap submission was done before that, and
during the review/merge of the patch, this rename was forgotten.

This commit therefore fixes the c-icap dependency on BR2_STATIC_LIBS,
which should fix:

  http://autobuild.buildroot.net/results/cb623460de30dd7c4ef3275fe13220c1ca0642a5/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>