build: Fix installation of generated headers

The `install_dir` argument is relative to the prefix, not to the include
directory specified in the `includedir` command line argument.
3 jobs for install-gen-header in 9 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #23271
ubuntu-16.04

00:09:01

passed #23272
ubuntu-17.10

00:09:31

passed #23273
ubuntu-18.04

00:09:24