network-manager: sync kernel headers comment with actual dependency

In commit 52282828
("package/network-manager: add patches to build with kernel headers
3.2"), network-manager was changed to build with kernel headers as old
as 3.2, instead of 3.7 before. The dependency was changed accordingly,
but the Config.in comment was not.

This commit fixes this inconsistency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>