Add err.h include to tests

lite.h no longer includes err.h it has to be included in each test.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

tests/fexist.c

@@ -1,3 +1,4 @@
+#include <err.h>
 #include "check.h"
 
 int main(void)

tests/fisdir.c

@@ -1,3 +1,4 @@
+#include <err.h>
 #include "check.h"
 
 int main(void)

tests/pidfile.c

@@ -1,3 +1,4 @@
+#include <err.h>
 #include <paths.h>
 #include <signal.h>
 #include <stdarg.h>