package/zlib-ng: Conditionally enable Power8 option

zlib-ng's build system does not correctly detect if it supports the
Power8 feature. Force it off to fix building for configurations that
don't support the vector builtin functions.

For example, building for BR2_powerpc_601:

  warning: implicit declaration of function ‘vec_xl’; did you mean
 ‘vec_rl’? [-Wimplicit-function-declaration]
   93 |             vbuf = vec_xl(0, (unsigned char *) buf);

This logic was incorrectly removed in commit 04e26cb7feb.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/zlib-ng/zlib-ng.mk

@@ -23,4 +23,11 @@ ifeq ($(BR2_arm),y)
 ZLIB_NG_CONF_OPTS += -DWITH_ACLE=1 -DWITH_NEON=1
 endif
 
+ifeq ($(BR2_powerpc_power8),y)
+ZLIB_NG_CONF_OPTS += -DWITH_POWER8=ON
+else
+ZLIB_NG_CONF_OPTS += -DWITH_POWER8=OFF
+endif
+
+
 $(eval $(cmake-package))