tests: lib: custom_lib: fix includes

Test was including Kernel for nothing, and missed limits.h
(likely included by Kernel)

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

tests/lib/custom_lib/src/main.c

@@ -11,8 +11,10 @@
  * library works correctly.
  */
 
-#include <zephyr/kernel.h>
+#include <limits.h>
+
 #include <zephyr/ztest.h>
+
 #include <custom_lib/custom_lib.h>
 
 ZTEST(custom_lib, test_get_value)