linuxkpi: Add pm_runtime_get_if_active() variant from Linux 6.9

With Linux 6.9, pm_runtime_get_if_active() lost its second bool
argument.

This change is put behind a check of LINUXKPI_VERSION to keep
compatibility.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50991