Add _SC_NSIG.

https://pubs.opengroup.org/onlinepubs/9799919799.2024edition/functions/sysconf.html (2024) says "Austin Group Defect 741 is applied, adding _SC_NSIG".

Change-Id: Ic8372105bac153403ab41369805c46322c25013e
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index 3143c23..c28a46e 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -1000,6 +1000,9 @@
   VERIFY_SYSCONF_POSITIVE(_SC_EXPR_NEST_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_LINE_MAX);
   VerifySysconf(_SC_NGROUPS_MAX, "_SC_NGROUPS_MAX", [](long v){return v >= 0 && v <= NGROUPS_MAX;});
+#if defined(__BIONIC__) || defined(_SC_NSIG) // New in POSIX 2024.
+  EXPECT_EQ(NSIG, sysconf(_SC_NSIG));
+#endif
   VERIFY_SYSCONF_POSITIVE(_SC_OPEN_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_PASS_MAX);
   VERIFY_SYSCONF_POSIX_VERSION(_SC_2_C_BIND);