package/iputils: use relative symlink for ping6

This was initially commit 84d471a0b150bc2ff1d08755d36be86d9380c2c5,
but it got mistakenly reverted by
95b0078cc089b48be8973b9507599cc836397869, so let's restore it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/iputils/iputils.mk

@@ -61,7 +61,7 @@ endif
 
 # upstream requires distros to create symlink
 define IPUTILS_CREATE_PING6_SYMLINK
-	ln -sf $(TARGET_DIR)/bin/ping $(TARGET_DIR)/bin/ping6
+	ln -sf ping $(TARGET_DIR)/bin/ping6
 endef
 IPUTILS_POST_INSTALL_TARGET_HOOKS += IPUTILS_CREATE_PING6_SYMLINK