Fixes missing header issue for C99 compatibility.
https://github.com/stewartsmith/libeatmydata/issues/29

diff --git a/libeatmydata/test/tst-invalidfd.c b/libeatmydata/test/tst-invalidfd.c
index c7df7a3..2aa60a9 100644
--- a/libeatmydata/test/tst-invalidfd.c
+++ b/libeatmydata/test/tst-invalidfd.c
@@ -20,6 +20,11 @@
 #include <unistd.h>
 #include <assert.h>
 
+#ifdef HAVE_SYNC_FILE_RANGE
+#define _GNU_SOURCE
+#include <fcntl.h>
+#endif
+
 int main()
 {
 	int fd = 42;