hash.h: Avoid compiler warnings with MSVC.

The lack of 'const' in function declaration causes MSVC to complain
because the function definition uses it.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Ben Pfaff <blp@nicira.com>

lib/hash.h

@@ -36,8 +36,9 @@ uint32_t hash_bytes(const void *, size_t n_bytes, uint32_t basis);
 
 static inline uint32_t hash_int(uint32_t x, uint32_t basis);
 static inline uint32_t hash_2words(uint32_t, uint32_t);
-static inline uint32_t hash_uint64(uint64_t);
-static inline uint32_t hash_uint64_basis(uint64_t x, uint32_t basis);
+static inline uint32_t hash_uint64(const uint64_t);
+static inline uint32_t hash_uint64_basis(const uint64_t x,
+                                         const uint32_t basis);
 uint32_t hash_3words(uint32_t, uint32_t, uint32_t);
 
 static inline uint32_t hash_boolean(bool x, uint32_t basis);