ruby.h: set rb_event_flag_t to uint32_t

This ensures a consistent implementation between 32 and 64-bit
platforms, as vm_trace.c limits events to 32-bits.  This can
also open up struct packing opportunities.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47759 b2dd03c8-39d4-4d8f-98ff-823fe69b080e