Fix CentOS 6 featur with libstdcpp

/usr/src/fastnetmon/fastnetmon.cpp: In function ‘void execute_ip_ban(uint32_t, map_element, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, std::string)’:
/usr/src/fastnetmon/fastnetmon.cpp:2333: ошибка: вызов перегруженной функции ‘abs(unsigned int)’ неоднозначен
/usr/include/stdlib.h:771: замечание: претенденты: int abs(int)
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/cmath:102: замечание:              long double std::abs(long double)
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/cmath:98: замечание:              float std::abs(float)
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/cmath:94: замечание:              double std::abs(double)
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/cstdlib:170: замечание:              long long int __gnu_cxx::abs(long long int)
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/cstdlib:139: замечание:              long int std::abs(long int)
make[2]: *** [CMakeFiles/fastnetmon.dir/fastnetmon.cpp.o] Ошибка 1
make[1]: *** [CMakeFiles/fastnetmon.dir/all] Ошибка 2