std.c: Also make Sigaction flags a c_uint for serenity

This matches all other platforms. Even if this field is defined as 'int'
in the C definition, the expectation is that the full 32-bit unsigned
integer range can be used. In particular this Sigaction initializer in
the new std.debug code was causing a build failure:

```zig
.flags = (posix.SA.SIGINFO | posix.SA.RESTART | posix.SA.RESETHAND)
```

Author: linusg

lib/std/c.zig

@@ -3355,15 +3355,15 @@ pub const Sigaction = switch (native_os) {
     },
     // https://github.com/SerenityOS/serenity/blob/ec492a1a0819e6239ea44156825c4ee7234ca3db/Kernel/API/POSIX/signal.h#L39-L46
     .serenity => extern struct {
-        pub const handler_fn = *align(1) const fn (c_int) callconv(.c) void;
-        pub const sigaction_fn = *const fn (c_int, *const siginfo_t, ?*anyopaque) callconv(.c) void;
+        pub const handler_fn = *align(1) const fn (i32) callconv(.c) void;
+        pub const sigaction_fn = *const fn (i32, *const siginfo_t, ?*anyopaque) callconv(.c) void;
 
         handler: extern union {
             handler: ?handler_fn,
             sigaction: ?sigaction_fn,
         },
         mask: sigset_t,
-        flags: c_int,
+        flags: c_uint,
     },
     else => void,
 };