better install for eventchannel support (now only 1 installer)

This follows this commit: ossec/ossec-hids@75a91043c3d64cd2a7e5dcbb077755bf2aa85760

This commit modifies the build process of the Windows installer in order to have only one installer handle two cases:

  - Deploy ossec-agent-eventchannel.exe on Vista or greater
  - Deploy ossec-agent.exe otherwise

The installer packages the two executables and checks Windows version at runtime in order to decide which version of "ossec-agent.exe" should be used.