posix: time.h: timepsec is a struct not a typedef

A previous typo went undetected that declared a type-defined
`timespec_t`, which is obviously incorrect, since it is only specified as
`struct timespec`.

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

Author: cfriedt

include/zephyr/posix/posix_time.h

@@ -61,10 +61,10 @@ struct sigevent;
 /* struct timespec must be defined in the libc time.h */
 #else
 #if !defined(_TIMESPEC_DECLARED) && !defined(__timespec_defined)
-typedef struct {
+struct timespec {
 	time_t tv_sec;
 	long tv_nsec;
-} timespec_t;
+};
 #define _TIMESPEC_DECLARED
 #define __timespec_defined
 #endif