swupdate: add upstream patch to fix build error

When building SWUpdate with the following defconfig:

```
CONFIG_DOWNLOAD=y
```

.. the build process breaks with:

```
corelib/channel_curl.c:27:10: fatal error: json-c/json.h: No such file or directory
 #include <json-c/json.h>
```

Looking at the SWUpdate Kconfig based build system shows that `CONFIG_DOWNLOAD`
depends on `HAVE_LIBCURL`, which selects CURL, which eventually enables the
(unnecessary) build of channel_curl.o.

The upstream fixes the condition for building channel_curl.o by adding a new
hidden config option `CHANNEL_CURL`, which is only selected by the
dependent options.

Backported from:
https://github.com/sbabic/swupdate/commit/37a6666a532e9cbc42b56301f27919ae7c00d2eb



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>