Fixes the config.h include error during build:

```console
In file included from src/xprintidle-ng.c:36:
lib/time.h:28:3: error: #error "Please include config.h first."
    28 |  #error "Please include config.h first."
      |   ^~~~~
```

Introduced since gnulib upgrade:
* https://github.com/coreutils/gnulib/commit/8ad7bc6
* https://lists.gnu.org/archive/html/bug-gnulib/2023-04/msg00088.html
* https://www.gnu.org/software/gnulib/manual/html_node/Source-changes.html

The last version of gnulib now checks that config.h is the first include
in every compilation unit.

This is not the case with the xprintidle-ng source code, so this patch
moves these `config.h` inclusions to be first.

---
 src/time-format.c   | 2 +-
 src/xprintidle-ng.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/time-format.c b/src/time-format.c
index 3a46749..4c0a4d2 100644
--- a/src/time-format.c
+++ b/src/time-format.c
@@ -18,10 +18,10 @@
 //     51 Franklin Street, Fifth Floor
 //     Boston, MA 02110-1301, USA.

+#include <config.h>
 #include <stdbool.h>
 #include <signal.h>
 #include <time.h>
-#include <config.h>
 #include "system.h"
 #include "xalloc.h"

diff --git a/src/xprintidle-ng.c b/src/xprintidle-ng.c
index 5156adf..ca69b2d 100644
--- a/src/xprintidle-ng.c
+++ b/src/xprintidle-ng.c
@@ -28,13 +28,13 @@
 // Eivind Magnus Hvidevold <hvidevold@gmail.com>.
 // kpowersave is licensed under the GNU GPL, version 2 _only_.

+#include <config.h>
 #include <X11/Xlib.h>
 #include <X11/extensions/dpms.h>
 #include <X11/extensions/scrnsaver.h>
 #include <stdbool.h>
 #include <signal.h>
 #include <time.h>
-#include <config.h>
 #include "system.h"
 #include "errno.h"
 #include "error.h"
--
2.42.0