Fix linking of utils::to_string

I've found the following error when compiling head (f8af59e):

    src/controller.o: In function `newsbeuter::controller::reload_indexes(std::vector<int, std::allocator<int> > const&, bool)':
    /home/ivan/build/newsbeuter/src/controller.cpp:790: undefined reference to `std::string newsbeuter::utils::to_string<unsigned int>(unsigned int)'
    /home/ivan/build/newsbeuter/src/controller.cpp:791: undefined reference to `std::string newsbeuter::utils::to_string<unsigned int>(unsigned int)'
    /home/ivan/build/newsbeuter/src/controller.cpp:792: undefined reference to `std::string newsbeuter::utils::to_string<unsigned int>(unsigned int)'
    /home/ivan/build/newsbeuter/src/controller.cpp:793: undefined reference to `std::string newsbeuter::utils::to_string<unsigned int>(unsigned int)'
    src/controller.o: In function `newsbeuter::controller::reload_all(bool)':
    /home/ivan/build/newsbeuter/src/controller.cpp:915: undefined reference to `std::string   newsbeuter::utils::to_string<unsigned int>(unsigned int)'
    src/controller.o:/home/ivan/build/newsbeuter/src/controller.cpp:916: more undefined   references to `std::string newsbeuter::utils::to_string<unsigned int>(unsigned int)' follow
    collect2: error: ld returned 1 exit status
    make: ** [newsbeuter] Erro 1

My system:
 - linux 3.8.4
 - g++ 4.8.0
 - glibc 2.17

Adding an instance of utils::to_string with type unsigned int fixed it.