mmc: Add a prototype for board_mmc_init()

Fixes the following sparse warning:

wandboard.c:137:5: warning: symbol 'board_mmc_init' was not declared. Should it be static?

Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>

include/mmc.h

@@ -349,4 +349,6 @@ struct mmc *mmc_spi_init(uint bus, uint cs, uint speed, uint mode);
 int mmc_legacy_init(int verbose);
 #endif
 
+int board_mmc_init(bd_t *bis);
+
 #endif /* _MMC_H_ */