patch 8.1.1414: alloc() returning "char_u *" causes a lot of type casts

Problem:    Alloc() returning "char_u *" causes a lot of type casts.
Solution:   Have it return "void *". (Mike Williams)  Define ALLOC_ONE() to
            check the simple allocations.
diff --git a/src/gui_gtk.c b/src/gui_gtk.c
index 8537060..a881aad 100644
--- a/src/gui_gtk.c
+++ b/src/gui_gtk.c
@@ -1452,7 +1452,7 @@
 	if (*p == DLG_BUTTON_SEP)
 	    ++count;
 
-    array = (char **)alloc((count + 1) * sizeof(char *));
+    array = ALLOC_MULT(char *, count + 1);
     count = 0;
 
     if (array != NULL)