roscpp: Fix clashing namespaces for ros::param::set

Compiling on RHEL with gcc 4.4.6 resulted in the following error:
```
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_set.h: In function 'void ros::param::setImpl(const std::string&, const std::vector<T, std::allocator<_CharT> >&)':
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_set.h:87: error: 'template<class _Key, class _Compare, class _Alloc> class std::set' is not a function,
/home/codac-dev/rpmbuild/BUILD/ros-hydro-core-1.9.50/src/roscpp/include/ros/param.h:161: error:   conflict with 'void ros::param::set(const std::string&, const std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool> > >&)'
```
Apparently the set was clashing with std::set for some reason.

Signed-off-by: Ruben Smits <ruben.smits@intermodalics.eu>