[platform-core-dev] changes to plugin manifest files

[DJ Houghton <DJ_Houghton@xxxxxxxxxx>]

As per bug 36590, I have added the "eclipse" pre-condition tag to all the
Core projects and fragments. Please catch up to these changes when merging
code for the next build.

Specifically, I have changed the following projects:
org.apache.xerces
org.eclipse.core.boot
org.eclipse.core.runtime
org.eclipse.core.resources
org.eclipse.core.resources.win32
org.eclipse.core.resources.linux
org.eclipse.core.resources.hpux
org.eclipse.core.resources.macosx
org.eclipse.core.resources.qnx
org.eclipse.core.tests.harness
org.eclipse.core.tests.runtime
org.eclipse.core.tests.resources
org.eclipse.pde.build
org.eclipse.webdav