root: Add maven-eclipse-plugin generated stuff to .gitignore

Using recent version of maven-eclipse-plugin (root pom.xml
doesn't specify <version> for this plugin!), I've ended up
having some extra generated stuff that should be ignored
by git.

Seems like .externalToolBuilders + maven-eclipse.xml are
generated due to <resource> declarations in pom.xml files,
i.e. maven-eclipse-plugin adds custom Ant builder for
Eclipse IDE for purpose of Maven <resource> processing.

Change-Id: I8863bafd25b5dc5dc45378939b7e54adc727ce74
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>