[osboot] Update framebuffer format to match new magenta constants

See magenta commit 4f0f431.

Change-Id: I42fb2d43cbb6719144aa28fa91dc4446d0b45adc
diff --git a/src/osboot.c b/src/osboot.c
index fc01d3c..684f50c 100644
--- a/src/osboot.c
+++ b/src/osboot.c
@@ -331,7 +331,7 @@
     ZP32(kernel.zeropage, ZP_FB_WIDTH) = (UINT32)gop->Mode->Info->HorizontalResolution;
     ZP32(kernel.zeropage, ZP_FB_HEIGHT) = (UINT32)gop->Mode->Info->VerticalResolution;
     ZP32(kernel.zeropage, ZP_FB_STRIDE) = (UINT32)gop->Mode->Info->PixelsPerScanLine;
-    ZP32(kernel.zeropage, ZP_FB_FORMAT) = 4; // XRGB32
+    ZP32(kernel.zeropage, ZP_FB_FORMAT) = 5; // XRGB32
     ZP32(kernel.zeropage, ZP_FB_REGBASE) = 0;
     ZP32(kernel.zeropage, ZP_FB_SIZE) = 256 * 1024 * 1024;