compat: always include linux/compat.h from net/compat.h

We're about to do reshuffling in networking headers and
eliminate some implicit includes. This results in:

In file included from ../net/ipv4/netfilter/arp_tables.c:26:
include/net/compat.h:60:40: error: unknown type name ‘compat_uptr_t’; did you mean ‘compat_ptr_ioctl’?
    struct sockaddr __user **save_addr, compat_uptr_t *ptr,
                                        ^~~~~~~~~~~~~
                                        compat_ptr_ioctl
include/net/compat.h:61:4: error: unknown type name ‘compat_size_t’; did you mean ‘compat_sigset_t’?
    compat_size_t *len);
    ^~~~~~~~~~~~~
    compat_sigset_t

Currently net/compat.h depends on linux/compat.h being included
first. After the upcoming changes this would break the 32bit build.
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20201121214844.1488283-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/compat.h

@@ -5,8 +5,6 @@
 
 struct sock;
 
-#if defined(CONFIG_COMPAT)
-
 #include <linux/compat.h>
 
 struct compat_msghdr {
@@ -48,14 +46,6 @@ struct compat_rtentry {
 	unsigned short  rt_irtt;        /* Initial RTT                  */
 };
 
-#else /* defined(CONFIG_COMPAT) */
-/*
- * To avoid compiler warnings:
- */
-#define compat_msghdr	msghdr
-#define compat_mmsghdr	mmsghdr
-#endif /* defined(CONFIG_COMPAT) */
-
 int __get_compat_msghdr(struct msghdr *kmsg, struct compat_msghdr __user *umsg,
 			struct sockaddr __user **save_addr, compat_uptr_t *ptr,
 			compat_size_t *len);