Add compile definition to indicate using of libtorrent 2.0 (#15297)

The compile definition is temporary which will be removed when qbt
ditches libtorrent 1.x.

src/CMakeLists.txt

@@ -36,6 +36,7 @@ endmacro()
 find_libtorrent(${minLibtorrent1Version})
 if (LibtorrentRasterbar_FOUND AND (LibtorrentRasterbar_VERSION VERSION_GREATER_EQUAL 2.0))
     find_libtorrent(${minLibtorrentVersion})
+    target_compile_definitions(qbt_common_cfg PUBLIC QBT_USES_LIBTORRENT2)
 endif()
 
 # force variable type so that it always shows up in ccmake/cmake-gui frontends