iperf: fix out of tree configuration

The configure step uses a package header to check for support
of types/declares, this package header encapsulates all the
required system headers for providing the needed resources.
When configured from an out of tree directory the package
header is not found due to the hard path.
We now make the path for package header relative to 'srcdir' so
it is found appropriately.

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>