Help the build system find system RocksDB.

Taken from this upstream commit:
https://github.com/ceph/ceph/commit/2ac26bd0e01fd6c82bd59936cf25c25173f7775a

diff --git a/cmake/modules/Findrocksdb.cmake b/cmake/modules/Findrocksdb.cmake
index f8369f73fc..5926647348 100644
--- a/cmake/modules/Findrocksdb.cmake
+++ b/cmake/modules/Findrocksdb.cmake
@@ -15,11 +15,11 @@ find_library(ROCKSDB_LIBRARIES rocksdb)
 
 if(ROCKSDB_INCLUDE_DIR AND EXISTS "${ROCKSDB_INCLUDE_DIR}/rocksdb/version.h")
   foreach(ver "MAJOR" "MINOR" "PATCH")
-    file(STRINGS "${ROCKSDB_INCLUDE_DIR}/version.h" ROCKSDB_VER_${ver}_LINE
+    file(STRINGS "${ROCKSDB_INCLUDE_DIR}/rocksdb/version.h" ROCKSDB_VER_${ver}_LINE
       REGEX "^#define[ \t]+ROCKSDB_${ver}[ \t]+[0-9]+$")
     string(REGEX REPLACE "^#define[ \t]+ROCKSDB_${ver}[ \t]+([0-9]+)$"
-      "\\1" ROCKSDB_VERSION_${ver} "${ROCKDB_VER_${ver}_LINE}")
-    unset(${ROCKDB_VER_${ver}_LINE})
+      "\\1" ROCKSDB_VERSION_${ver} "${ROCKSDB_VER_${ver}_LINE}")
+    unset(${ROCKSDB_VER_${ver}_LINE})
   endforeach()
   set(ROCKSDB_VERSION_STRING
     "${ROCKSDB_VERSION_MAJOR}.${ROCKSDB_VERSION_MINOR}.${ROCKSDB_VERSION_PATCH}")