riscv: Wire up memfd_secret in UAPI header

Move the __ARCH_WANT_MEMFD_SECRET define added in commit 7bb7f2ac


("arch, mm: wire up memfd_secret system call where relevant") to
<uapi/asm/unistd.h> so __NR_memfd_secret is defined when including
<unistd.h> in userspace.

This allows the memfd_secret selftest to pass on riscv.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20220505081815.22808-1-tklauser@distanz.ch
Fixes: 7bb7f2ac

 ("arch, mm: wire up memfd_secret system call where relevant")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>