Stop relying on header pollution in sdhci_xenon.c

Include sys/malloc.h directly in sdhci_xenon.c to get the malloc(9)
definition rather than depend on header pollution.

Sponsored by:	The FreeBSD Foundation

sys/dev/sdhci/sdhci_xenon.c

@@ -37,6 +37,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/bus.h>
 #include <sys/kernel.h>
 #include <sys/lock.h>
+#include <sys/malloc.h>
 #include <sys/module.h>
 #include <sys/mutex.h>
 #include <sys/resource.h>