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>
--- cups-1.3.11/Makedefs.in.orig	2014-03-05 13:28:10.175437880 +0000
+++ cups-1.3.11/Makedefs.in	2014-03-05 13:28:35.398845162 +0000
@@ -132,7 +132,7 @@ LDFLAGS		=	-L../cups -L../filter @LDARCH
 LEGACY_BACKENDS	=	@LEGACY_BACKENDS@
 LIBCUPSORDER	=	@LIBCUPSORDER@
 LIBCUPSIMAGEORDER =	@LIBCUPSIMAGEORDER@
-LINKCUPS	=	@LINKCUPS@ $(SSLLIBS)
+LINKCUPS	=	@LINKCUPS@ $(SSLLIBS) $(LIBZ)
 LINKCUPSIMAGE	=	@LINKCUPSIMAGE@
 LIBS		=	$(LINKCUPS) $(COMMONLIBS)
 OPTIM		=	@OPTIM@