webkit: fix mmu reverse dependency moving it to %_ARCH_SUPPORTS

Because mmu dependency was not propagated move it to
BR2_PACKAGE_WEBKIT_ARCH_SUPPORTS to automatically propagate it.
It can ben moved to BR2_PACKAGE_WEBKIT_ARCH_SUPPORTS because like
target architecture dependencies it doesn't modify the comment string.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>