gsoap: add missing dependency on zlib

* do_compile fails without:
| i586-oe-linux-g++  -m32 -march=i586 --sysroot=/OE/sysroots/qemux86  -O2 -pipe -g -feliminate-unused-debug-types -fpermissive -fvisibility-inlines-hidden -L. -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed -o wsdl2h wsdl2h-wsdl2h.o wsdl2h-wsdl.o wsdl2h-schema.o wsdl2h-types.o wsdl2h-service.o wsdl2h-soap.o wsdl2h-mime.o wsdl2h-wsp.o wsdl2h-wsdlC.o ../../gsoap/libgsoapssl++.a -lssl -lcrypto -lz
| /OE/sysroots/x86_64-linux/usr/libexec/i586-oe-linux/gcc/i586-oe-linux/4.7.2/ld: cannot find -lz
| collect2: error: ld returned 1 exit status

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>