swupdate: add upstream patch to fix musl build issue

Add upstream patch to fix build issue with the musl C library, as musl
does not provide the GNU extension `strndupa()` breaking the build with
an undefined reference:

```
ipc/lib.a(network_ipc.o): In function `ipc_postupdate':
network_ipc.c:(.text.ipc_postupdate+0x39): undefined reference to `strndupa'
```

The upstream patch provides a compatibility header file adding a definition
of `strndupa` if it is not already defined.

Backported from:
https://github.com/sbabic/swupdate/commit/9867a9d6a21e6b0b9bcba57c3e2398fe671cea17

Fixes:
http://autobuild.buildroot.net/results/f674219225d0b67d0bc78fd10b47a865250746f4/
http://autobuild.buildroot.net/results/d2b0cfcfec583d203af1b26152f14ded3ebd3eee/
http://autobuild.buildroot.net/results/1d0cbd1eb9b1b70bd9f4d1c4fae431baee9d105c/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>