mm/hmm: fix build when HMM is disabled

Combinatorial Kconfig is painfull. Withi this patch all below combination
build.

1)

2)
CONFIG_HMM_MIRROR=y

3)
CONFIG_DEVICE_PRIVATE=y

4)
CONFIG_DEVICE_PUBLIC=y

5)
CONFIG_HMM_MIRROR=y
CONFIG_DEVICE_PUBLIC=y

6)
CONFIG_HMM_MIRROR=y
CONFIG_DEVICE_PRIVATE=y

7)
CONFIG_DEVICE_PRIVATE=y
CONFIG_DEVICE_PUBLIC=y

8)
CONFIG_HMM_MIRROR=y
CONFIG_DEVICE_PRIVATE=y
CONFIG_DEVICE_PUBLIC=y

Link: http://lkml.kernel.org/r/[e-mail]
Reported-by: Randy Dunlap <[e-mail]>
Signed-off-by: Jérôme Glisse <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/hmm.h

···
 struct hmm_device *hmm_device_new(void *drvdata);
 void hmm_device_put(struct hmm_device *hmm_device);
 #endif /* CONFIG_DEVICE_PRIVATE || CONFIG_DEVICE_PUBLIC */
-
+#endif /* IS_ENABLED(CONFIG_HMM) */
 
 /* Below are for HMM internal use only! Not to be used by device driver! */
 #if IS_ENABLED(CONFIG_HMM_MIRROR)
···
 #else /* IS_ENABLED(CONFIG_HMM) */
 static inline void hmm_mm_destroy(struct mm_struct *mm) {}
 static inline void hmm_mm_init(struct mm_struct *mm) {}
-#endif /* IS_ENABLED(CONFIG_HMM) */
 #endif /* LINUX_HMM_H */