ruby: explicitly disable dtrace support

both 2.2.1 and 2.2.2 are failing on some hosts native builds with:
| linking miniruby
| load.o: In function `rb_f_load':
| load.c:(.text+0x3af): undefined reference to `RUBY_DTRACE_LOAD_ENTRY_ENABLED'
| load.c:(.text+0x428): undefined reference to `RUBY_DTRACE_LOAD_RETURN_ENABLED'
| load.c:(.text+0x452): undefined reference to `RUBY_DTRACE_LOAD_RETURN'
| load.c:(.text+0x492): undefined reference to `RUBY_DTRACE_LOAD_ENTRY'
| load.o: In function `rb_require_internal':

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