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/libc/bionic/sysconf.cpp b/libc/bionic/sysconf.cpp
index 571370c..03822ed 100644
--- a/libc/bionic/sysconf.cpp
+++ b/libc/bionic/sysconf.cpp
@@ -210,6 +210,7 @@
       // are vestigial anyway, so the "maximum maximum" of NGROUPS_MAX is a good
       // enough answer for _SC_NGROUPS_MAX...
       return NGROUPS_MAX;
+    case _SC_NSIG:              return NSIG;
     case _SC_PASS_MAX:          return PASS_MAX;
     case _SC_2_C_BIND:          return _POSIX2_C_BIND;
     case _SC_2_C_DEV:           return _POSIX2_C_DEV;