Standalone Apps: Standalone apps should need only exports.h.

Modify exports.h to remove its dependencies on other files, thus
enabling standalone apps to require only exports.h from the U-Boot
source tree.  This appears to be the intent based on the following
note: http://lists.denx.de/pipermail/u-boot/2010-January/067174.html

Signed-off-by: Mike Partington <mparting@lexmark.com>

examples/standalone/atmel_df_pow2.c

@@ -8,6 +8,7 @@
 
 #include <common.h>
 #include <exports.h>
+#include <spi.h>
 
 #define CMD_ID    0x9f
 #define CMD_STAT  0xd7

examples/standalone/stubs.c

@@ -1,3 +1,4 @@
+#include <common.h>
 #include <exports.h>
 
 #ifndef GCC_VERSION