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/cmd/usb.c b/cmd/usb.c
index dd9ac0b..98fa300 100644
--- a/cmd/usb.c
+++ b/cmd/usb.c
@@ -11,6 +11,7 @@
  */
 
 #include <common.h>
+#include <blk.h>
 #include <command.h>
 #include <console.h>
 #include <dm.h>