Fix for GCC 5.4 (#1157)

GCC 5.4 will complain (see also options_parser.cc):

    /home/abuild/rpmbuild/BUILD/arangodb-3.0.0r1/3rdParty/rocksdb/rocksdb/util/options_builder.cc: In function 'rocksdb::CompactionStyle rocksdb::{anonymous}::PickCompactionStyle(size_t, int, int, uint64_t)':
    /home/abuild/rpmbuild/BUILD/arangodb-3.0.0r1/3rdParty/rocksdb/rocksdb/util/options_builder.cc:29:7: error: 'log' is not a member of 'std'
           std::log(target_db_size / write_buffer_size) / std::log(kBytesForLevelMultiplier)));
           ^
    /home/abuild/rpmbuild/BUILD/arangodb-3.0.0r1/3rdParty/rocksdb/rocksdb/util/options_builder.cc:29:7: note: suggested alternative:
    In file included from /usr/include/features.h:365:0,
                     from /usr/include/math.h:26,
                     from /home/abuild/rpmbuild/BUILD/arangodb-3.0.0r1/3rdParty/rocksdb/rocksdb/util/options_builder.cc:6:
    /usr/include/bits/mathcalls.h:109:1: note:   'log'
     __MATHCALL_VEC (log,, (_Mdouble_ __x));

util/options_builder.cc

@@ -4,6 +4,7 @@
 //  of patent rights can be found in the PATENTS file in the same directory.
 
 #include <math.h>
+#include <cmath>
 #include <algorithm>
 #include "rocksdb/options.h"