ruby: requires shared library

No matter what you do, the Ruby build system wants to use shared
libraries for the encoding modules. Therefore, this commit disallows
the selection of Ruby in BR2_PREFER_STATIC_LIB situations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>