patch 9.0.0063: too many type casts for dict_get functions

Problem:    Too many type casts for dict_get functions.
Solution:   Change the key argument from "char_u *" to "char *".
diff --git a/src/quickfix.c b/src/quickfix.c
index c37caa5..49484e0 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -7209,18 +7209,18 @@
     if (first_entry)
 	did_bufnr_emsg = FALSE;
 
-    filename = dict_get_string(d, (char_u *)"filename", TRUE);
-    module = dict_get_string(d, (char_u *)"module", TRUE);
-    bufnum = (int)dict_get_number(d, (char_u *)"bufnr");
-    lnum = (int)dict_get_number(d, (char_u *)"lnum");
-    end_lnum = (int)dict_get_number(d, (char_u *)"end_lnum");
-    col = (int)dict_get_number(d, (char_u *)"col");
-    end_col = (int)dict_get_number(d, (char_u *)"end_col");
-    vcol = (int)dict_get_number(d, (char_u *)"vcol");
-    nr = (int)dict_get_number(d, (char_u *)"nr");
-    type = dict_get_string(d, (char_u *)"type", TRUE);
-    pattern = dict_get_string(d, (char_u *)"pattern", TRUE);
-    text = dict_get_string(d, (char_u *)"text", TRUE);
+    filename = dict_get_string(d, "filename", TRUE);
+    module = dict_get_string(d, "module", TRUE);
+    bufnum = (int)dict_get_number(d, "bufnr");
+    lnum = (int)dict_get_number(d, "lnum");
+    end_lnum = (int)dict_get_number(d, "end_lnum");
+    col = (int)dict_get_number(d, "col");
+    end_col = (int)dict_get_number(d, "end_col");
+    vcol = (int)dict_get_number(d, "vcol");
+    nr = (int)dict_get_number(d, "nr");
+    type = dict_get_string(d, "type", TRUE);
+    pattern = dict_get_string(d, "pattern", TRUE);
+    text = dict_get_string(d, "text", TRUE);
     if (text == NULL)
 	text = vim_strsave((char_u *)"");
 
@@ -7243,7 +7243,7 @@
 
     // If the 'valid' field is present it overrules the detected value.
     if (dict_has_key(d, "valid"))
-	valid = (int)dict_get_bool(d, (char_u *)"valid", FALSE);
+	valid = (int)dict_get_bool(d, "valid", FALSE);
 
     status =  qf_add_entry(qfl,
 			NULL,		// dir
@@ -7419,7 +7419,7 @@
 	return FAIL;
 
     vim_free(qfl->qf_title);
-    qfl->qf_title = dict_get_string(what, (char_u *)"title", TRUE);
+    qfl->qf_title = dict_get_string(what, "title", TRUE);
     if (qf_idx == qi->qf_curlist)
 	qf_update_win_titlevar(qi);