digsig.h: 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>

include/linux/digsig.h

@@ -29,7 +29,7 @@ struct pubkey_hdr {
 	uint32_t	timestamp;	/* key made, always 0 for now */
 	uint8_t		algo;
 	uint8_t		nmpi;
-	char		mpi[0];
+	char		mpi[];
 } __packed;
 
 struct signature_hdr {
@@ -39,7 +39,7 @@ struct signature_hdr {
 	uint8_t		hash;
 	uint8_t		keyid[8];
 	uint8_t		nmpi;
-	char		mpi[0];
+	char		mpi[];
 } __packed;
 
 #if defined(CONFIG_SIGNATURE) || defined(CONFIG_SIGNATURE_MODULE)