gnutls.inc: set libz prefix

* without this patch host's zlib is used, similar to issue with
  libreadline it's important if host's zlib is compatible with MACHINE
  and in this case zlib was in DEPENDS so header was always available in
  sysroot
  lib/config.log:
  configure:18288: checking whether to include zlib compression support
  configure:18291: result: yes
  configure:18786: checking for libz
  configure:18808: x86_64-oe-linux-gcc    -m64 --sysroot=/OE/shr-core/tmp-eglibc/sysroots/qemux86-64 -o conftest -O2 -pipe -g -feliminate-unused-debug-types -I/OE/shr-core/tmp-eglibc/sysroots/qemux86-64/usr/include -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed conftest.c /usr/lib/libz.so  >&5
  configure:18808: $? = 0
  configure:18818: result: yes
  configure:18825: checking how to link with libz
  configure:18827: result: /usr/lib/libz.so

(From OE-Core rev: 6d21c87ca37a11b34551d79758da65ed417c70a0)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-support/gnutls/gnutls.inc

@@ -3,7 +3,7 @@ HOMEPAGE = "http://www.gnu.org/software/gnutls/"
 BUGTRACKER = "https://savannah.gnu.org/support/?group=gnutls"
 DEPENDS = "zlib lzo libtasn1 libgcrypt (>= 1.4.2) libcap readline"
 
-INC_PR = "r3"
+INC_PR = "r4"
 
 LICENSE = "GPLv3+ & LGPLv2.1+"
 LICENSE_${PN} = "LGPLv2.1+"
@@ -25,6 +25,7 @@ EXTRA_OECONF="--with-included-opencdk --with-included-libcfg --disable-rpath \
               --with-libdl-prefix=${STAGING_DIR_HOST}${prefix} \
               --with-libpthread-prefix=${STAGING_DIR_HOST}${prefix} \
               --with-libreadline-prefix=${STAGING_DIR_HOST}${prefix} \
+              --with-libz-prefix=${STAGING_DIR_HOST}${prefix} \
               --with-lzo --disable-guile \
               --without-p11-kit \
               "