libqmi: fix build when ARPHRD_RAWIP isn't defined in libc
The kernel may be new enough to have rmnet support, but it could be that libc doesn't have ARPHRD_RAWIP defined yet; if so, just define it ourselves.
Fixes http://autobuild.buildroot.org/results/c8853b7161bd85d46c1282c6c097e7ef4042ae68