posix: signal.h: reorder where sigval and siginfo_t are declared

In order to avoid warnings, declare `union sigval` ahead of
`struct sigevent` and declare `siginfo_t` ahead of `struct sigaction`.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

include/zephyr/posix/posix_signal.h

diff --git a/include/zephyr/posix/posix_signal.h b/include/zephyr/posix/posix_signal.h
index 3e23fdf..d0e8907 100644
--- a/include/zephyr/posix/posix_signal.h
+++ b/include/zephyr/posix/posix_signal.h
@@ -98,7 +98,15 @@
 
 #if defined(_POSIX_REALTIME_SIGNALS) || defined(__DOXYGEN__)
 
-union sigval; /* forward declaration (to preserve spec order) */
+/* slightly out of order w.r.t. the specification */
+#if !defined(_SIGVAL_DECLARED) && !defined(__sigval_defined)
+union sigval {
+	int sival_int;
+	void *sival_ptr;
+};
+#define _SIGVAL_DECLARED
+#define __sigval_defined
+#endif
 
 #if !defined(_SIGEVENT_DECLARED) && !defined(__sigevent_defined)
 struct sigevent {
@@ -121,17 +129,28 @@
 
 #endif /* defined(_POSIX_REALTIME_SIGNALS) || defined(__DOXYGEN__) */
 
-#if !defined(_SIGVAL_DECLARED) && !defined(__sigval_defined)
-union sigval {
-	int sival_int;
-	void *sival_ptr;
-};
-#define _SIGVAL_DECLARED
-#define __sigval_defined
-#endif
-
 /* SIGRTMIN and SIGRTMAX defined above */
 
+#if !defined(_SIGINFO_T_DECLARED) && !defined(__siginfo_t_defined)
+typedef struct {
+	void *si_addr;
+#if defined(_XOPEN_STREAMS) || defined(__DOXYGEN__)
+	long si_band;
+#endif
+	union sigval si_value;
+	pid_t si_pid;
+	uid_t si_uid;
+	int si_signo;
+	int si_code;
+#if defined(_XOPEN_SOURCE) || defined(__DOXYGEN__)
+	int si_errno;
+#endif
+	int si_status;
+} siginfo_t;
+#define _SIGINFO_T_DECLARED
+#define __siginfo_t_defined
+#endif
+
 #if defined(_POSIX_REALTIME_SIGNALS) || defined(__DOXYGEN__)
 
 #if !defined(_SIGACTION_DECLARED) && !defined(__sigaction_defined)
@@ -200,26 +219,6 @@
 
 #endif /* defined(_POSIX_REALTIME_SIGNALS) || defined(__DOXYGEN__) */
 
-#if !defined(_SIGINFO_T_DECLARED) && !defined(__siginfo_t_defined)
-typedef struct {
-	void *si_addr;
-#if defined(_XOPEN_STREAMS) || defined(__DOXYGEN__)
-	long si_band;
-#endif
-	union sigval si_value;
-	pid_t si_pid;
-	uid_t si_uid;
-	int si_signo;
-	int si_code;
-#if defined(_XOPEN_SOURCE) || defined(__DOXYGEN__)
-	int si_errno;
-#endif
-	int si_status;
-} siginfo_t;
-#define _SIGINFO_T_DECLARED
-#define __siginfo_t_defined
-#endif
-
 /* Siginfo codes are defined below */
 
 #if !defined(_SIGHANDLER_T_DECLARED) && !defined(__sighandler_t_defined)