webkit: Restrict supported architectures

See Source/WTF/wtf/Platform.h. Webkit is only supported for
arm, armeb, i386, mips, mipsel, powerpc, sh, sparc, x86_64.

[Thomas P: propagate dependency to the midori package.]

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>