Misc: common nanosecond clock impl, Files.exists() pre-impl for linux

core/src/main/c/share/files.c

@@ -278,3 +278,8 @@ JNIEXPORT jboolean JNICALL Java_com_questdb_std_Files_rename
         (JNIEnv *e, jclass cls, jlong lpszOld, jlong lpszNew) {
     return (jboolean) (rename((const char *) lpszOld, (const char *) lpszNew) == 0);
 }
+
+JNIEXPORT jboolean JNICALL Java_com_questdb_std_Files_exists0
+        (JNIEnv *e, jclass cls, jlong lpsz) {
+    return access((const char *) lpsz, F_OK);
+}

core/src/main/c/share/os.c

@@ -27,6 +27,7 @@
 #include <sys/errno.h>
 #include <stdlib.h>
 #include <sys/time.h>
+#include <time.h>
 #include "../share/os.h"
 
 JNIEXPORT jint JNICALL Java_com_questdb_std_Os_getPid