Resolve some MSVC warnings by including missing header files

src/conio/win32cio.c

@@ -21,6 +21,8 @@
 #include <windows.h>	/* INPUT_RECORD, etc. */
 #include <genwrap.h>
 #include <stdio.h>		/* stdin */
+#include <stdlib.h>		/* atexit */
+
 #if defined(_WIN32)
  #include <malloc.h>	/* alloca() on Win32 */
 #endif

src/conio/win32gdi.c

@@ -4,6 +4,7 @@
 #include <math.h>
 #include <stdbool.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 #include <genwrap.h>
 #include <xp_dl.h>