ANSI C fixes.
Removed warnings.
Made the include files self-sufficient.
Some Solaris fixes.

diff --git a/windows/msgbox.c b/windows/msgbox.c
index 21a9207..ec5ff2b 100644
--- a/windows/msgbox.c
+++ b/windows/msgbox.c
@@ -5,6 +5,8 @@
  */
 
 #include <string.h>
+
+#include "wine/winbase16.h"
 #include "wine/winuser16.h"
 #include "dlgs.h"
 #include "heap.h"