patch 8.1.0912: MS-Windows: warning for signed/unsigned

Problem:    MS-Windows: warning for signed/unsigned.
Solution:   Add type cast. (Nobuhiro Takasaki, closes #3945)
diff --git a/src/terminal.c b/src/terminal.c
index 3be2698..a823499 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -5722,7 +5722,7 @@
     job->jv_proc_info = proc_info;
     job->jv_job_object = jo;
     job->jv_status = JOB_STARTED;
-    job->jv_tty_type = vim_strsave("conpty");
+    job->jv_tty_type = vim_strsave((char_u *)"conpty");
     ++job->jv_refcount;
     term->tl_job = job;
 
@@ -6056,7 +6056,7 @@
 	    (short_u *)winpty_conin_name(term->tl_winpty), NULL);
     job->jv_tty_out = utf16_to_enc(
 	    (short_u *)winpty_conout_name(term->tl_winpty), NULL);
-    job->jv_tty_type = vim_strsave("winpty");
+    job->jv_tty_type = vim_strsave((char_u *)"winpty");
     ++job->jv_refcount;
     term->tl_job = job;