deskutils/xcalendar: Fix LLVM15 build

At a minimum fix an incompatible pointer to integer conversion flagged
by LLVM15. While at it include stdlib.h in preparation for additional
work needed on this port.

While at all of the above, assume care and feeding of this port.

(cherry picked from commit 2a92344327b6550ce4978f331e5f1119c610e14d)