Replace FT_UNUSED with FC_UNUSED.

FreeType is trying to hide FT_UNUSED, so use FC_UNUSED instead.
diff --git a/src/ftglue.c b/src/ftglue.c
index 0e02bd2..7490a8c 100644
--- a/src/ftglue.c
+++ b/src/ftglue.c
@@ -229,13 +229,10 @@
 
     for ( nn = 0; nn < count; nn++ )
     {
-      FT_ULong  tag      = GET_ULong();
-      FT_ULong  checksum = GET_ULong();
-      FT_ULong  start    = GET_ULong();
-      FT_ULong  size     = GET_ULong();
-
-      FT_UNUSED(checksum);
-      FT_UNUSED(size);
+      FT_ULong  tag                = GET_ULong();
+      FT_ULong  checksum FC_UNUSED = GET_ULong();
+      FT_ULong  start              = GET_ULong();
+      FT_ULong  size     FC_UNUSED = GET_ULong();
 
       if ( tag == the_tag )
       {