include: expose clock constants to POSIX.1b consumers

[mihawk-99]

Code that selects _POSIX_C_SOURCE=199309L can see clock_gettime() in this SDK's <time.h>, but cannot use CLOCK_REALTIME: the constants are hidden until 200112. TIMER_ABSTIME has the same mismatch.

This showed up while porting code using libretro-common's threading helpers. Lowering the two visibility guards to 199309 lets those consumers compile without changing their feature-test macros. The numeric values and function declarations stay the same.

Current FreeBSD uses the same visibility level for these constants. In particular, its broader clock visibility is a compatibility choice; this PR does not claim every clock in the block was introduced in POSIX.1b.

Validation

Cross-compiled with Clang 23.1.1 targeting x86_64-sie-ps5. The following source fails against upstream and compiles with this change:

#define _POSIX_C_SOURCE 199309L
#include <time.h>

int clocks[] = {CLOCK_REALTIME, CLOCK_MONOTONIC, TIMER_ABSTIME};
int (*get_time)(clockid_t, struct timespec *) = clock_gettime;

From the repository root, save it as /tmp/sdk-clock-visibility.c and run:

clang -target x86_64-sie-ps5 -isystem include/freebsd \
    -fsyntax-only /tmp/sdk-clock-visibility.c

Also checked 199506L (fixed), 200112L, 200809L, and the default feature level (still compile), and 199209L (the constants remain hidden). This is a compile-time header fix; no console run was needed.