ruby: add missing dependency on ruby-native

* without ruby available on host it fails:
  | NOTE: make LIBRUBYARG=-lruby-static
  |       CC = i586-starfish-linux-gcc  -m32 -march=i586 --sysroot=/OE/sysroots/qemux86
  |       LD = i586-starfish-linux-ld --sysroot=/OE/sysroots/qemux86
  |       LDSHARED = i586-starfish-linux-gcc  -m32 -march=i586 --sysroot=/OE/sysroots/qemux86 -shared
  |       CFLAGS = -O2 -pipe -g -feliminate-unused-debug-types -fPIC
  |       XCFLAGS = -include ruby/config.h -include ruby/missing.h -fvisibility=hidden -DRUBY_EXPORT
  |       CPPFLAGS =   -I. -I.ext/include/i386-linux -I./include -I.
  |       DLDFLAGS = -Wl,-soname,libruby.so.1.9
  |       SOLIBS = -lpthread -lrt -ldl -lcrypt -lm
  | executable host ruby is required. use --with-baseruby option.
  | make: *** [.rbconfig.time] Error 1

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>