LibC: unistd.h should provide SEEK_SET etc. if stdio.h is not included (#629)

Seems POSIX is odd and unistd also defines SEEK_SET if stdio.h
This simply follows the pattern several other C libraries take of setting
these macros to the same values in stdio if that header is not included.

Libraries/LibC/unistd.h

@@ -1,3 +1,10 @@
+/* standard symbolic constants and types
+ *
+ * values from POSIX standard unix specification
+ *
+ * https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html
+ */
+
 #pragma once
 
 #include <errno.h>
@@ -12,6 +19,14 @@ __BEGIN_DECLS
 #define STDOUT_FILENO 1
 #define STDERR_FILENO 2
 
+/* lseek whence values */
+#ifndef _STDIO_H /* also defined in stdio.h */
+#define SEEK_SET 0 /* from beginning of file.  */
+#define SEEK_CUR 1 /* from current position in file.  */
+#define SEEK_END 2 /* from the end of the file.  */
+#endif
+
+
 extern char** environ;
 
 int get_process_name(char* buffer, int buffer_size);