patch 8.0.0750: OpenPTY missing in non-GUI build

Problem:    OpenPTY missing in non-GUI build.
Solution:   Always include pty.c, add an #ifdef to skip over the contents.
diff --git a/src/pty.c b/src/pty.c
index 20ab65b..a738ac1 100644
--- a/src/pty.c
+++ b/src/pty.c
@@ -43,6 +43,8 @@
 
 #include "vim.h"
 
+#if defined(FEAT_GUI) || defined(FEAT_TERMINAL)
+
 #include <signal.h>
 
 #ifdef __CYGWIN32__
@@ -414,3 +416,5 @@
     return -1;
 }
 #endif
+
+#endif /* FEAT_GUI || FEAT_TERMINAL */