fix the build error(warning) in g++ 4.7.2

The error is the following:
control.cc:82:40: error: narrowing conversion of 'supported_compilers.std::vector<_Tp, _Alloc>::size<const Compiler*, std::allocator<const Compiler*> >()' from 'std::vector<const Compiler*>::size_type {aka long unsigned int}' to 'uint32_t {aka unsigned int}' inside { } is ill-formed in C++11 [-Werror=narrowing]