tests/timer: tweak path to config.h

When building out-of-srcdir, "../../config.h" fails to reach config.h
because the compiler is invoked in $builddir/tests/, not
$builddir/tests/timer/.  Use "../config.h" instead; this also works
for in-srcdir builds.

Signed-off-by: Alex Badea <vamposdecampos@gmail.com>
diff --git a/tests/timer/timer_test.c b/tests/timer/timer_test.c
index ba3127d..bb9a177 100644
--- a/tests/timer/timer_test.c
+++ b/tests/timer/timer_test.c
@@ -33,7 +33,7 @@
 #include <osmocom/core/select.h>
 #include <osmocom/core/linuxlist.h>
 
-#include "../../config.h"
+#include "../config.h"
 
 static void main_timer_fired(void *data);
 static void secondary_timer_fired(void *data);