common: Drop part.h from common header

Move this uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/spl.h b/include/spl.h
index 5a67c82..abd75c6 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -26,6 +26,7 @@
 #define MMCSD_MODE_FS		2
 #define MMCSD_MODE_EMMCBOOT	3
 
+struct blk_desc;
 struct image_header;
 
 /*