arm64: mm: explicity include linux/vmalloc.h

arm64's mm/mmu.c uses vm_area_add_early, struct vm_area and other
definitions  but relies on implict inclusion of linux/vmalloc.h which
means that changes in other headers could break the build. Thus, add an
explicit include.
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/mmu.c

@@ -31,6 +31,7 @@
 #include <linux/fs.h>
 #include <linux/io.h>
 #include <linux/mm.h>
+#include <linux/vmalloc.h>
 
 #include <asm/barrier.h>
 #include <asm/cputype.h>