libsecret: Update for 0.20.0

Needed for updating epiphany.

0.20.0
* secret-backend: New interface to represent password storage backend [!34]
* secret-backend: Add local-storage backend [!6]
* item: Port to GTask [!43]
* Build fixes [!34, !37, !38, !40, !41, !42, ...]
* Updated translations

0.19.1
* service: Fix secret_service_ensure_session_finish error propagation [!36]

0.19.0
* secret-password: Add necessary functions to migrate from D-Bus based API [!32]
* egg: Request that secure memory not be dumped to disk [!30]
* Add version macros [!29]
* Add missing GType to flags in .gir [!16, !19]
* paths: Port from GSimpleAsyncResult to GTask [!26]
* build: Bump meson_version to 0.50 [!18, !35]
* Build and test fixes [!15, !20, !21, !23, !33, ...]

(nia)