package/iprutils: add patch to fix musl build

This commit adds a patch to the iprutils to fix the build with the
musl C library.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: reformatted the patch with Git, added a better commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>