build(cmake): minimal fixes in CMakeLists.txt for transitive restbed headers, i2pcommon, and versioning

[jolavillette]

build(cmake): minimal fixes in CMakeLists.txt for transitive restbed headers, i2pcommon, and versioning

This PR stabilizes the CMake configuration of libretroshare to support downstream modular builds (such as retroshare-gui and retroshare-service).

To ensure maximum stability and ease of review, the modifications made to the CMakeLists files are kept strictly minimal and highly targeted:

• Transitive Headers (1 line change): Promoted the restbed include directory to PUBLIC in CMakeLists.txt so that downstream modules cleanly inherit the HTTP/JSON API headers without duplicate include paths.
• Link Fixes (2 lines change): Reactivated the compilation of util/i2pcommon.cpp/.h under CMake, resolving undefined reference linking errors in downstream GUI settings panels.
• Version Display (minimal additions):
  Directed the GUI human-readable version query to run in the parent directory's .git if present, ensuring the GUI gets the correct overall application version instead of submodule tags.
  Added the RS_LIB_VERSION_HASH compile definition on the core target, fixing the [version not available] display bug in the about box.

All changes are non-intrusive and strictly limited to resolving compile/link-time blockers.