Fix rocksdb cmake depends should use list instead of string

[git-hulk]

Kvrocks would throw below errors on debian 11 when building:

CMake Error at /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find JeMalloc (missing: JeMalloc_LIBRARIES JeMalloc_INCLUDE_DIRS)
Call Stack (most recent call first):
  /usr/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  cmake/modules/FindJeMalloc.cmake:17 (find_package_handle_standard_args)
  CMakeLists.txt:103 (find_package)

It's caused by the wrong type of rocksdb_DEPENDS.