move fallocate64 declaration under _LARGEFILE64_SOURCE feature test

overlooked in commit 25e6fee27f4a293728dd15b659170e7b9c7db9bc,
probably because the fallocate function itself is nonstandard and
already under _GNU_SOURCE.

include/fcntl.h

@@ -184,7 +184,6 @@ struct f_owner_ex {
 #define SPLICE_F_MORE 4
 #define SPLICE_F_GIFT 8
 int fallocate(int, int, off_t, off_t);
-#define fallocate64 fallocate
 int name_to_handle_at(int, const char *, struct file_handle *, int *, int);
 int open_by_handle_at(int, struct file_handle *, int);
 ssize_t readahead(int, off_t, size_t);
@@ -207,6 +206,9 @@ ssize_t tee(int, int, size_t, unsigned);
 #define posix_fadvise64 posix_fadvise
 #define posix_fallocate64 posix_fallocate
 #define off64_t off_t
+#if defined(_GNU_SOURCE)
+#define fallocate64 fallocate
+#endif
 #endif
 
 #ifdef __cplusplus