Fix build warning

[Smit-create]

/Users/thebigbool/repos/lpython/src/libasr/runtime/lfortran_intrinsics.c:735:11: warning: absolute value function 'abs' given an argument of type 'int64_t' (aka 'long long') but has parameter of type 'int' which may cause truncation of value [-Wabsolute-value]
    num = abs(num);