use std::abs from cstdlib which fixes a warning that occurs when compiling with clang 3.5 on 64-bit linux

in file included from ../include/spdlog/sinks/../formatter.h:57:
../include/spdlog/details/pattern_formatter_impl.h:308:26: warning: absolute value function 'abs' given an argument of type 'rep' (aka 'long') but has parameter of type 'int' which may cause truncation of value [-Wabsolute-value]
        auto secs_diff = abs((duration_cast<seconds>(diff)).count());
                         ^
../include/spdlog/details/pattern_formatter_impl.h:308:26: note: use function 'std::abs' instead
        auto secs_diff = abs((duration_cast<seconds>(diff)).count());
                         ^~~
                         std::abs
1 warning generated.