cups: fix static linking

Append -lz to the cups linking libs to fix a problem like this one:

../cups/libcups.a(file.o): In function `cupsFileSeek':
/home/test/test/1/output/build/cups-1.3.11/cups/file.c:1444: undefined
reference to `inflateEnd'
collect2: error: ld returned 1 exit status
make[2]: *** [ipp] Error 1
make[2]: *** Waiting for unfinished jobs....

This solution is the same used upstream in newer versions.

Fixes:
   http://autobuild.buildroot.net/results/f3d/f3d4f67ce7ee1d54f5bd4c87dfddb7417db614e5/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>