posix: signal.h: always pull in time_t definition from libc

Avoid redefining `time_t` by always including the definition from
`<time.h>`, since the C library may use something other than `long`.

This is still safe according to the specification, since it explicitly
states:

> Inclusion of the <signal.h> header may make visible all symbols from
> the <time.h> header.

For more information, please see
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

include/zephyr/posix/posix_signal.h

@@ -43,11 +43,8 @@ typedef int uid_t;
 #define __uid_t_defined
 #endif
 
-#if !defined(_TIME_T_DECLARED) && !defined(__time_t_defined)
-typedef long time_t;
-#define _TIME_T_DECLARED
-#define __time_t_defined
-#endif
+/* time_t must be defined by the libc time.h */
+#include <time.h>
 
 #if !defined(_TIMESPEC_DECLARED) && !defined(__timespec_defined)
 struct timespec {