RISC-V: Add missing include

Fixes:

include/asm-generic/mm_hooks.h:20:11: warning: 'struct vm_area_struct' declared inside parameter list will not be visible outside of this definition or declaration
include/asm-generic/mm_hooks.h:19:38: warning: 'struct mm_struct' declared inside parameter list will not be visible outside of this definition or declaration

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/mmu_context.h

@@ -14,6 +14,7 @@
 #ifndef _ASM_RISCV_MMU_CONTEXT_H
 #define _ASM_RISCV_MMU_CONTEXT_H
 
+#include <linux/mm_types.h>
 #include <asm-generic/mm_hooks.h>
 
 #include <linux/mm.h>