libifconfig: Don't install the header

[manu]

Summary

Since r344530 we've switch libifconfig as an internallib so it's not installed.
Do not install the header too.
This "solves" a problem with pkgbase where a libifconfig-development package
is created and due to how packages are created we add a dependency to a
libifconfig package that doesn't exists.

Reported by: pizzamig

Signed-off-by: Emmanuel Vadot <[e-mail]>

[emaste]

OK with me, with the same comment as the other #INCS review that I wonder if we can instead just skip headers when INTERNALLIB is set

[manu]

Better solution in D24166