use HsInt64 instead of int64_t, fixes this test on Windows (and is more correct)

testsuite/tests/ghc-regress/ccall/should_run/ffi018.h

+#include "HsFFI.h"
 
-#include <sys/types.h>
-
-int64_t f(void);
+HsInt64 f(void);
 

testsuite/tests/ghc-regress/ccall/should_run/ffi018_c.c

 
 #include "ffi018.h"
 
-int64_t f(void) {
+HsInt64 f(void) {
     return 0x123456789ABCDEF0LL;
 }