[Axis2] Renaming axis2_stax_ombuilder.h axis2_om_stax_builder.h

[Samisa Abeysinghe]

Hi Nandika
   I renamed axis2_stax_ombuilder.h to axis2_om_stax_builder.h so that 
we could be able to locate all OM related headers at a glance
Thanks,
Samisa...