std.c: Fix return type for NetBSD's __sigfillset14().

Author: alexrp

lib/std/c.zig

@@ -11263,7 +11263,7 @@ const private = struct {
     extern "c" fn __msync13(addr: *align(page_size) const anyopaque, len: usize, flags: c_int) c_int;
     extern "c" fn __nanosleep50(rqtp: *const timespec, rmtp: ?*timespec) c_int;
     extern "c" fn __sigaction14(sig: c_int, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) c_int;
-    extern "c" fn __sigfillset14(set: ?*sigset_t) void;
+    extern "c" fn __sigfillset14(set: ?*sigset_t) c_int;
     extern "c" fn __sigprocmask14(how: c_int, noalias set: ?*const sigset_t, noalias oset: ?*sigset_t) c_int;
     extern "c" fn __socket30(domain: c_uint, sock_type: c_uint, protocol: c_uint) c_int;
     extern "c" fn __stat50(path: [*:0]const u8, buf: *Stat) c_int;