c++tools: Include <cstdlib> for exit [PR100731]

This TU uses exit, but doesn't include <stdlib.h> or <cstdlib> and relies
on some other header to include it indirectly, which apparently doesn't
happen on reporter's host.

The other <c*> headers aren't guarded either and we rely on a compiler
capable of C++11, so maybe we can rely on <cstdlib> being around
unconditionally.

2021-05-25  Jakub Jelinek  <jakub@redhat.com>

	PR bootstrap/100731
	* server.cc: Include <cstdlib>.