cups: fix libdir handling

Cups uses /usr/lib64 if the HOST has it, no matter if it is cross compiling
for a 32bit arch, breaking the build for stuff looking in /usr/lib.

Fixes http://autobuild.buildroot.net/results/4ed4f5b43d05a40ab15f141442f15da56c87be3b



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>