Fix compilation failure due to missing FALLOC_FL_KEEP_SIZE

FALLOC_FL_KEEP_SIZE flag has existed since fallocate() was first
introduced, but it was not defined in glibc headers for a while.
Define it manually if necessary.

Follow-up b9db91e1 ("xlog: fix fallocate vs read race").

src/box/xlog.c

@@ -47,6 +47,17 @@
 #include "iproto_constants.h"
 #include "errinj.h"
 
+/*
+ * FALLOC_FL_KEEP_SIZE flag has existed since fallocate() was
+ * first introduced, but it was not defined in glibc headers
+ * for a while. Define it manually if necessary.
+ */
+#ifdef HAVE_FALLOCATE
+# ifndef FALLOC_FL_KEEP_SIZE
+#  define FALLOC_FL_KEEP_SIZE 0x01
+# endif
+#endif /* HAVE_FALLOCATE */
+
 /*
  * marker is MsgPack fixext2
  * +--------+--------+--------+--------+