platform: msm_shared: Fix compilation warnings in msm shared drivers.

Fix compilation warnings in shared drivers.

Change-Id: I7c73b60c4a9a38436ea0da5290fc0c9d5ebfa045
diff --git a/platform/msm_shared/include/boot_device.h b/platform/msm_shared/include/boot_device.h
index ad17f75..c216586 100644
--- a/platform/msm_shared/include/boot_device.h
+++ b/platform/msm_shared/include/boot_device.h
@@ -58,4 +58,6 @@
 uint32_t platform_get_boot_dev();
 uint32_t platform_boot_dev_isemmc();
 void platform_boot_dev_cmdline(char *buf);
+
+void *target_mmc_device();
 #endif