Input: gpio_keys_polled - 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>
Link: https://lore.kernel.org/r/20200214171907.GA26588@embeddedorSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/gpio_keys_polled.c

@@ -38,7 +38,7 @@ struct gpio_keys_polled_dev {
 	const struct gpio_keys_platform_data *pdata;
 	unsigned long rel_axis_seen[BITS_TO_LONGS(REL_CNT)];
 	unsigned long abs_axis_seen[BITS_TO_LONGS(ABS_CNT)];
-	struct gpio_keys_button_data data[0];
+	struct gpio_keys_button_data data[];
 };
 
 static void gpio_keys_button_event(struct input_dev *input,