QtLocation: copy private declarative headers during "make install"

At the moment the private declarative headers ("qtlocation/src/imports") have to be manually copied from the source package for creating custom declarative map items.
Example headers which are useful for custom extensions:

• qdeclarativegeomapitembase_p.h
• qdeclarativegeomapquickitem_p.h

They should be automatically copied (like the other private QtLocation headers) during "make install".