doc: mention SCMP_ARCH_LOONGARCH64 in seccomp_arch_add(3)

Just found out the missing bit while browsing the docs in preparation of
a pet project of mine.

Fixes: 6966ec7 ("arch: Add 64-bit LoongArch support")
Signed-off-by: WANG Xuerui <git@xen0n.name>
Acked-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

doc/man/man3/seccomp_arch_add.3

@@ -17,6 +17,7 @@ seccomp_arch_add, seccomp_arch_remove, seccomp_arch_exist, seccomp_arch_native \
 .B #define SCMP_ARCH_X32
 .B #define SCMP_ARCH_ARM
 .B #define SCMP_ARCH_AARCH64
+.B #define SCMP_ARCH_LOONGARCH64
 .B #define SCMP_ARCH_MIPS
 .B #define SCMP_ARCH_MIPS64
 .B #define SCMP_ARCH_MIPS64N32