lib: Include OstreeRemote and OstreeCollectionRef in GIR

lib: Include OstreeRemote and OstreeCollectionRef in GIR

Now that g-ir-scanner is being told about ENABLE_EXPERIMENTAL_API, it
can include these types correctly. Drop the __GI_SCANNER__ guards in the
header files so that all the declarations are found.

After this, you can actually construct the types normally:

>>> OSTree.CollectionRef.new('com.example.Foo', 'bar')
<OSTree.CollectionRef object at 0x7f2bba4c7528 (OstreeCollectionRef at 0x55c033ff2f30)>

Closes: #1337
Approved by: pwithnall