selftests/memfd: Fix unknown type name build failure

Partially backport v6.3 commit 11f75a01 ("selftests/memfd: add tests
for MFD_NOEXEC_SEAL MFD_EXEC") to fix an unknown type name build error.
In some systems, the __u64 typedef is not present due to differences in
system headers, causing compilation errors like this one:

fuse_test.c:64:8: error: unknown type name '__u64'
   64 | static __u64 mfd_assert_get_seals(int fd)

This header includes the  __u64 typedef which increases the likelihood
of successful compilation on a wider variety of systems.

Signed-off-by: Hardik Garg <hargar@linux.microsoft.com>
Reviewed-by: Tyler Hicks (Microsoft) <code@tyhicks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>