Add the opt-in annotations required for compiling metadata

`gradle clean check` is apparently not enough to detect all
compilation problems, one must also do `gradle publishToMavenLocal`
to be sure.

Author: dkhalanskyjb

core/native/src/Instant.kt

@@ -131,7 +131,7 @@ private const val MAX_SECOND = 31494816403199L // +1000000-12-31T23:59:59
 private fun isValidInstantSecond(second: Long) = second >= MIN_SECOND && second <= MAX_SECOND
 
 
-@OptIn(ExperimentalForeignApi::class)
+@OptIn(ExperimentalForeignApi::class, UnsafeNumber::class)
 internal fun currentTime(): Instant = memScoped {
     val tm = alloc<timespec>()
     val error = clock_gettime(CLOCK_REALTIME.convert(), tm.ptr)

core/nix/src/internal/filesystem.kt

@@ -3,7 +3,7 @@
  * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
  */
 
-@file:OptIn(ExperimentalForeignApi::class)
+@file:OptIn(ExperimentalForeignApi::class, UnsafeNumber::class)
 package kotlinx.datetime.internal
 
 import kotlinx.cinterop.*