Cygwin: pty: Move function hook_api() into hookapi.cc.

- PTY uses Win32 API hook for pseudo console suppot. The function
  hook_api() is used for this purpose and defined in fhandler_tty.cc
  previously. This patch moves it into hookapi.cc.

winsup/cygwin/winsup.h

@@ -199,6 +199,7 @@ ino_t __reg2 hash_path_name (ino_t hash, const char *name);
 void __reg2 nofinalslash (const char *src, char *dst);
 
 void __reg3 *hook_or_detect_cygwin (const char *, const void *, WORD&, HANDLE h = NULL);
+void __reg3 *hook_api (const char *mname, const char *name, const void *fn);
 
 /* Time related */
 void __stdcall totimeval (struct timeval *, PLARGE_INTEGER, int, int);