make brace placement in public header struct definitions consistent

placing the opening brace on the same line as the struct keyword/tag
is the style I prefer and seems to be the prevailing practice in more
recent additions.

these changes were generated by the command:

find include/ arch/*/bits -name '*.h' \
-exec sed -i '/^struct [^;{]*$/{N;s/\n/ /;}' {} +

and subsequently checked by hand to ensure that the regex did not pick
up any false positives.

arch/generic/bits/msg.h

@@ -1,5 +1,4 @@
-struct msqid_ds
-{
+struct msqid_ds {
 	struct ipc_perm msg_perm;
 	time_t msg_stime;
 	int __unused1;