fixed clang++ compile problem

in clang version 3.3 :

[100%] Building CXX object 3rd/CMakeFiles/test.dir/t.cc.obj
In file included from /home/andy/w/test/mesos/3rd/t.cc:7:
In file included from /home/andy/w/libprocess/include/process/async.hpp:4:
In file included from /home/andy/w/libprocess/include/process/dispatch.hpp:9:
In file included from /home/andy/w/libprocess/include/process/process.hpp:13:
/home/andy/w/libprocess/include/process/event.hpp:15:1: error: struct 'ProcessBase' was previously declared as a class [-Werror,-Wmismatched-tags]
struct ProcessBase;
^
/home/andy/w/libprocess/include/process/pid.hpp:14:7: note: previous use is here
class ProcessBase;
      ^
In file included from /home/andy/w/test/mesos/3rd/t.cc:7:
In file included from /home/andy/w/libprocess/include/process/async.hpp:4:
In file included from /home/andy/w/libprocess/include/process/dispatch.hpp:9:
/home/andy/w/libprocess/include/process/process.hpp:25:1: error: 'ProcessBase' defined as a class here but previously declared as a struct [-Werror,-Wmismatched-tags]
class ProcessBase : public EventVisitor
^
/home/andy/w/libprocess/include/process/event.hpp:15:1: note: did you mean class here?
struct ProcessBase;
^~~~~~
class