Fix misimport of ktor variable in common module

[ArnyminerZ]

HrefListProperty currently uses the ktor module:

https://github.com/bitfireAT/dav4jvm/blob/dfe4fa169fda071da635615b1c388d5165196d5c/src/main/kotlin/at/bitfire/dav4jvm/property/common/HrefListProperty.kt#L13

Which I believe is simply an error. We should have some common property somewhere outside both modules. Because obviously, HREF exists both in ktor:

https://github.com/bitfireAT/dav4jvm/blob/dfe4fa169fda071da635615b1c388d5165196d5c/src/main/kotlin/at/bitfire/dav4jvm/ktor/DavResource.kt#L103

and okhttp:

https://github.com/bitfireAT/dav4jvm/blob/main/src/main/kotlin/at/bitfire/dav4jvm/okhttp/DavResource.kt#L83

This forces applications that use okhttp to also include ktor's dependencies.