MAINTAINERS: adjust header file entry in DPLL SUBSYSTEM

Commit 9431063a ("dpll: core: Add DPLL framework base functions") adds
the section DPLL SUBSYSTEM in MAINTAINERS and includes a file entry to the
non-existing file 'include/net/dpll.h'.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference. Looking at the file stat of the commit above, this entry
clearly intended to refer to 'include/linux/dpll.h'.

Adjust this header file entry in DPLL SUBSYSTEM.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>