ruby: Fix build dependency of gmp/host

libgmp.so is needed for miniruby, that is compiled in host.
Using PKG_BUILD_DEPENDS=gmp/host eventually compiled gmp/host
before ruby/host. However, when this did not happened, build
failed. Using HOST_BUILD_DEPENDS=gmp/host seems to be the
correct way to require it.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>