package/pipewire: needs headers >= 3.18

package/pipewire: needs headers >= 3.18

v4l2 plugin needs headers >= 3.18 since
https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/4cb90f3b868e5169cb9bfe2200f3b079d3f0db7b
(so since its addition to buildroot in commit
75c86f90c73c42ee35559610aec28a02190b65b7) because of
V4L2_PIX_FMT_ARGB555X which is only available since
https://github.com/torvalds/linux/commit/fcc0d3db28922f9ba21ea6c7b23ea10ffb5d3521

v4l2 plugin can't be disabled until
https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/8d71d2dab831b77cadb74f2e4630f549acc94ac4

Fixes:
 - http://autobuild.buildroot.org/results/b887b6ccd2c22bb3214c07d1281ad486438fb58e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>