Fix waring: using abs() for floating point type

src/flowgrind.c:1336:10: warning: using integer absolute value function 'abs'
when argument is of floating point type [-Wabsolute-value]
        while ((abs(value) / (dez - 1.0)) > 1.0) {
                ^
src/flowgrind.c:1336:10: note: use function 'fabs' instead
        while ((abs(value) / (dez - 1.0)) > 1.0) {
                ^~~
                fabs