gdiplus: Print the GdipCreateHalftonePalette FIXME only once.

dlls/gdiplus/graphics.c

diff --git a/dlls/gdiplus/graphics.c b/dlls/gdiplus/graphics.c
index 18dae2d..7510fa0 100644
--- a/dlls/gdiplus/graphics.c
+++ b/dlls/gdiplus/graphics.c
@@ -4481,7 +4481,12 @@
 
 HPALETTE WINGDIPAPI GdipCreateHalftonePalette(void)
 {
-    FIXME("\n");
+    static int calls;
+
+    TRACE("\n");
+
+    if (!calls++)
+      FIXME("stub\n");
 
     return NULL;
 }