Treat RocksDB headers as 'system'
Treat RocksDB headers as 'system'

We are not in control of RocksDB's headers; as such any warnings they
present we basically cannot do anything about. on macOS the following
warning is printed when including <rocksdb/db.h>:

    tlm/deps/rocksdb.exploded/include/rocksdb/db.h:56:6: warning: redeclaration of already-defined enum 'EntryType' is a GNU extension [-Wgnu-redeclared-enum]
    enum EntryType;
         ^
    tlm/deps/rocksdb.exploded/include/rocksdb/types.h:20:6: note: previous definition is here
    enum EntryType {
         ^

As such, silence these warnings by classifying RocksDB headers as
system headers.

Change-Id: I6bcda0bd81e2d4cf2f711d18781d02527975b90c
Reviewed-on: http://review.couchbase.org/102212
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine