if prototype is declared static make function static to match

Today, a compiler was encountered that considered it an error
to have the prototype declared static and the function body
not

src/isaac64.c

@@ -19,7 +19,9 @@
 #define ISAAC64_MASK ((uint64_t)0xFFFFFFFFFFFFFFFFULL)
 
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L)
+#if !defined(HAS_INLINE)
 #define HAS_INLINE
+#endif
 #else
 # if (defined(__GNUC__) && __GNUC__ >= 2 && !defined(inline))
 # define inline __inline__