MFC r274900:

Fix the following -Werror warnings from clang 3.5.0, while building
bsnmpd's snmp_hostres module:

usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:204:20: error: absolute value function 'abs' given an argument of type 'const long' but has parameter of type 'int' which may cause truncation of value [-Werror,-Wabsolute-value]

str[9] = (u_char)(abs(tm->tm_gmtoff) / 3600);
                  ^

usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:204:20: note: use function 'labs' instead

str[9] = (u_char)(abs(tm->tm_gmtoff) / 3600);
                  ^~~
                  labs

usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:205:22: error: absolute value function 'abs' given an argument of type 'const long' but has parameter of type 'int' which may cause truncation of value [-Werror,-Wabsolute-value]

str[10] = (u_char)((abs(tm->tm_gmtoff) % 3600) / 60);
                    ^

usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:205:22: note: use function 'labs' instead

str[10] = (u_char)((abs(tm->tm_gmtoff) % 3600) / 60);
                    ^~~
                    labs

Since tm::tm_gmtoff is a long, use labs(3) instead.