io_uring/register: guard compat syscall with CONFIG_COMPAT

Add compat.h include to avoid a potential build issue:

io_uring/register.c:281:6: error: call to undeclared function 'in_compat_syscall'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]

if (in_compat_syscall()) {
    ^
1 warning generated.
io_uring/register.c:282:9: error: call to undeclared function 'compat_get_bitmap'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
ret = compat_get_bitmap(cpumask_bits(new_mask),
      ^

Fixes: c4320315 ("io_uring/register: move io_uring_register(2) related code to register.c")
Reported-by: Manu Bretelle <chantra@meta.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/register.c

@@ -14,6 +14,7 @@
 #include <linux/slab.h>
 #include <linux/uaccess.h>
 #include <linux/nospec.h>
+#include <linux/compat.h>
 #include <linux/io_uring.h>
 #include <linux/io_uring_types.h>
 
@@ -278,13 +279,14 @@ static __cold int io_register_iowq_aff(struct io_ring_ctx *ctx,
 	if (len > cpumask_size())
 		len = cpumask_size();
 
-	if (in_compat_syscall()) {
+#ifdef CONFIG_COMPAT
+	if (in_compat_syscall())
 		ret = compat_get_bitmap(cpumask_bits(new_mask),
 					(const compat_ulong_t __user *)arg,
 					len * 8 /* CHAR_BIT */);
-	} else {
+	else
+#endif
 		ret = copy_from_user(new_mask, arg, len);
-	}
 
 	if (ret) {
 		free_cpumask_var(new_mask);