package/swupdate: add patch to fix link error for CONFIG_UBOOT and missing libz

package/swupdate: add patch to fix link error for CONFIG_UBOOT and missing libz

CONFIG_UBOOT needs libz [1]. So CONFIG_UBOUT mustn't be available when
HAVE_ZLIB=n.

Add patch from upstream to fix a link error for CONFIG_UBOOT and missing libz.
Note, that this link error does not affect the default config provided by
Buildroot.

[1] https://github.com/sbabic/swupdate/commit/cf495e23ad117f4cae9c429e80714fe9c18f2d6a

[Thomas: use a single <pkg>_PATCH variable.]

Reported-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>