omap3: board: add missing include and proto

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
diff --git a/arch/arm/cpu/armv7/omap3/board.c b/arch/arm/cpu/armv7/omap3/board.c
index 08f8363..a1c5b09 100644
--- a/arch/arm/cpu/armv7/omap3/board.c
+++ b/arch/arm/cpu/armv7/omap3/board.c
@@ -17,6 +17,7 @@
  * SPDX-License-Identifier:	GPL-2.0+
  */
 #include <common.h>
+#include <mmc.h>
 #include <spl.h>
 #include <asm/io.h>
 #include <asm/arch/sys_proto.h>