convert_swig's -s option produces uncompilable code for STLs instantiated in namespaces

[dbankieris]

With TRICK_CONVERT_SWIG_FLAGS set to -s:

#include <vector>

namespace a { 

class A { 
    std::vector<int> v;
};

}

SWIG 2:

error: ‘std’ in namespace ‘a’ does not name a type
 SWIGINTERN a::std::vector< int >::size_type std_vector_Sl_int_Sg____len__(std::vector< int > const *self){
            ^

SWIG 4:

Error: 'std::vector' resolves to 'std::vector' and was incorrectly instantiated in scope 'a' instead of within scope 'std'.

This is due to #768