Don't export the unused __use_pts() routine.

The __use_pts() routine was once probably used by libutil to determine
if we are using BSD or UNIX98 style PTY device names. It doesn't seem to
be used outside grantpt.c, which means we can make it static and remove
it from the Symbol.map.

Reviewed by:	cognet, kib
Approved by:	philip (mentor)

lib/libc/stdlib/Symbol.map

@@ -94,7 +94,6 @@ FBSD_1.0 {
 };
 
 FBSDprivate_1.0 {
-	__use_pts;
 	_malloc_prefork;
 	_malloc_postfork;
 	__system;

lib/libc/stdlib/grantpt.c

@@ -73,7 +73,7 @@ __FBSDID("$FreeBSD$");
  */
 #define _PATH_PTCHOWN	"/usr/libexec/pt_chown"
 
-int
+static int
 __use_pts(void)
 {
 	int use_pts;