use <signal.h> instead of glibc header to enhance portability (musl)

building using musl fails with:

```
FEX/Source/Tools/LinuxEmulation/LinuxSyscalls/ThreadManager.h:33:10: fatal error: 'bits/types/sigset_t.h' file not found
   33 | #include <bits/types/sigset_t.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

`<bits/types/sigset_t.h>` is a glibc internal header, use <signal.h> instead.

fixes: #5459
Signed-off-by: Pepper Gray <hello@peppergray.xyz>

Source/Tools/LinuxEmulation/LinuxSyscalls/ThreadManager.h

@@ -30,7 +30,7 @@ $end_info$
 #include <optional>
 #include <sys/stat.h>
 
-#include <bits/types/sigset_t.h>
+#include <signal.h>
 #include <linux/seccomp.h>
 
 namespace FEX::HLE {