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/socket.h

@@ -1,5 +1,4 @@
-struct msghdr
-{
+struct msghdr {
 	void *msg_name;
 	socklen_t msg_namelen;
 	struct iovec *msg_iov;
@@ -9,8 +8,7 @@ struct msghdr
 	int msg_flags;
 };
 
-struct cmsghdr
-{
+struct cmsghdr {
 	socklen_t cmsg_len;
 	int cmsg_level;
 	int cmsg_type;