add missing posix_spawnattr_init/destroy functions

src/process/posix_spawnattr_destroy.c

+#include <spawn.h>
+
+int posix_spawnattr_destroy(posix_spawnattr_t *attr)
+{
+	return 0;
+}

src/process/posix_spawnattr_init.c

+#include <spawn.h>
+
+int posix_spawnattr_init(posix_spawnattr_t *attr)
+{
+	*attr = (posix_spawnattr_t){ 0 };
+	return 0;
+}