icu: fix build after custom data library patch

Commit abded6bf ("icu: add an option
to add a custom data library file") broke the build because of an
improper test on a non-qstripped variable. This commit fixes that.

Fixes:

  http://autobuild.buildroot.org/results/02a/02a0800b01a4bf8734d601f79dc12663fe8f7542/


  and many other similar occurences of the same issue.

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