efi/apple-properties: 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
inadvertenly introduced[3] to the codebase from now on.

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/20200211231421.GA15697@embeddedorSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/apple-properties.c

@@ -31,7 +31,7 @@ __setup("dump_apple_properties", dump_properties_enable);
 struct dev_header {
 	u32 len;
 	u32 prop_count;
-	struct efi_dev_path path[0];
+	struct efi_dev_path path[];
 	/*
 	 * followed by key/value pairs, each key and value preceded by u32 len,
 	 * len includes itself, value may be empty (in which case its len is 4)
@@ -42,7 +42,7 @@ struct properties_header {
 	u32 len;
 	u32 version;
 	u32 dev_count;
-	struct dev_header dev_header[0];
+	struct dev_header dev_header[];
 };
 
 static void __init unmarshal_key_value_pairs(struct dev_header *dev_header,