Add a few missing includes for config.h

This caused the headers to not haved _GNU_SOURCE set which in turn
caused clang-tidy to complain because util-strings.h didn't have
strtod_l.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1175>

src/timer.h

@@ -24,6 +24,8 @@
 #ifndef TIMER_H
 #define TIMER_H
 
+#include "config.h"
+
 #include <stdint.h>
 
 #include "libinput-util.h"

src/util-multivalue.h

@@ -24,6 +24,8 @@
 
 #pragma once
 
+#include "config.h"
+
 #include <stdint.h>
 #include <stdbool.h>
 

tools/shared.h

@@ -24,6 +24,8 @@
 #ifndef _SHARED_H_
 #define _SHARED_H_
 
+#include "config.h"
+
 #include <stdbool.h>
 #include <limits.h>
 #include <getopt.h>