drm/vmwgfx: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c

@@ -69,7 +69,7 @@ struct vmw_bo_dirty {
 	unsigned int ref_count;
 	unsigned long bitmap_size;
 	size_t size;
-	unsigned long bitmap[0];
+	unsigned long bitmap[];
 };
 
 /**

drivers/gpu/drm/vmwgfx/vmwgfx_surface.c

@@ -79,7 +79,7 @@ struct vmw_surface_dirty {
 	struct svga3dsurface_cache cache;
 	size_t size;
 	u32 num_subres;
-	SVGA3dBox boxes[0];
+	SVGA3dBox boxes[];
 };
 
 static void vmw_user_surface_free(struct vmw_resource *res);