Add a missing declaration of Log in lldb.

Bill accepted this commit for the 3.4 release.

The build was failing with:
/tmp/buildd/llvm-toolchain-3.4-3.4~+/tools/lldb/source/Host/common/Host.cpp:1222:30: error: request for member 'Printf' in 'log', which is of non-class type 'double(double)throw ()'
log->Printf("Host::GetLLDBPath(ePathTypeLLDBTempSystemDir) => '%s'", g_lldb_tmp_dir.GetCString());

llvm-svn: 197376