mmc: sdhci: fix build warning

If CONFIG_MMC_SDHCI_IO_ACCESSORS is defined, the following warning would
shows up:

include/sdhci.h:224: warning: 'struct sdhci_host' declared inside
parameter list
include/sdhci.h:224: warning: its scope is only this definition or
declaration, which is probably not what you want
include/sdhci.h:225: warning: 'struct sdhci_host' declared inside
parameter list
include/sdhci.h:226: warning: 'struct sdhci_host' declared inside
parameter list
include/sdhci.h:227: warning: 'struct sdhci_host' declared inside
parameter list
include/sdhci.h:228: warning: 'struct sdhci_host' declared inside
parameter list
include/sdhci.h:229: warning: 'struct sdhci_host' declared inside
parameter list

Signed-off-by: Lei Wen <leiwen@marvell.com>

include/sdhci.h

@@ -214,6 +214,9 @@
  */
 #define SDHCI_QUIRK_32BIT_DMA_ADDR	(1 << 0)
 
+/* to make gcc happy */
+struct sdhci_host;
+
 /*
  * Host SDMA buffer boundary. Valid values from 4K to 512K in powers of 2.
  */