fix build on recent musl (stat64 compat)

musl removed LFS64 compat[1] so stat64 is no longer defined by default,
but we can bring it back for now through _LARGEFILE64_SOURCE

Link: https://www.openwall.com/lists/musl/2022/09/26/1 [1]
Fixes: #446

src/libfaketime.c

@@ -23,6 +23,8 @@
 
 #define _GNU_SOURCE             /* required to get RTLD_NEXT defined */
 
+#define _LARGEFILE64_SOURCE 1   /* required for stat64 on musl */
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>