mmc: add missing prototype for mmc_get_env_dev

This is a follow-up patch to e92029c0 and adds a prototype for
the weak mmc_get_env_dev function.

Cc: Tom Rini <trini@konsulko.com>
Cc: Stephen Warren <swarren@nvidia.com>

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>

include/mmc.h

@@ -479,6 +479,7 @@ void board_mmc_power_init(void);
 int board_mmc_init(bd_t *bis);
 int cpu_mmc_init(bd_t *bis);
 int mmc_get_env_addr(struct mmc *mmc, int copy, u32 *env_addr);
+int mmc_get_env_dev(void);
 
 struct pci_device_id;