Include process.h on Windows for _exit

Fixes a recently introduced build error on Windows.

Microsoft's docs say to include either <process.h> or <stdlib.h>. I
arbitrarily chose process.h.

c++/src/kj/test-helpers.c++

@@ -24,6 +24,8 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#else
+#include <process.h>
 #endif
 
 namespace kj {