package/zlib-ng: handle power9 cpu, fix compile error

Zlib-ng misdetects the powerpc cpu and the package fails to compile
for non-power9 cpu's.
Power9 support was added Upstream in commit:
02d10b252cc54159f7c33823048daec4b023fb22
So it was introduced in zlib-ng 2.1.3 and this was added to Buildroot
in commit 0df456ea6e05306fb077e96064adf68f3863eeea.

So there is no need to backport it to older Buildroot releases.

Fixes:
 - http://autobuild.buildroot.net/results/a9f/a9f45486664b2d5b23a2f330a63955a06ae8189d

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/zlib-ng/zlib-ng.mk

@@ -35,4 +35,10 @@ else
 ZLIB_NG_CONF_OPTS += -DWITH_POWER8=OFF
 endif
 
+ifeq ($(BR2_powerpc_power9),y)
+ZLIB_NG_CONF_OPTS += -DWITH_POWER9=ON
+else
+ZLIB_NG_CONF_OPTS += -DWITH_POWER9=OFF
+endif
+
 $(eval $(cmake-package))