linux-musl-s390x: Add SYS_mseal

SYS_mseal is not yet defined in musl-libc, but it can only take one
possible value, dictated by the kernel's syscall ABI.

(backport <#4549>)
(cherry picked from commit 6a13fd5)

Author: neuschaefer

src/unix/linux_like/linux/musl/b64/s390x.rs

@@ -731,3 +731,4 @@ pub const SYS_futex_waitv: c_long = 449;
 pub const SYS_set_mempolicy_home_node: c_long = 450;
 pub const SYS_cachestat: c_long = 451;
 pub const SYS_fchmodat2: c_long = 452;
+pub const SYS_mseal: c_long = 462;