Fix Alpine Linux builds

Reported by Maxime Bouton.

* spot/misc/tmpfile.cc: Include stdlib.h, not cstdlib, so
that our replacement secure_getenv() is found.
* THANKS: Add Maxime.

THANKS

@@ -27,6 +27,7 @@ Juraj Major
 Kristin Y. Rozier
 Martin Dieguez Lodeiro
 Matthias Heizmann
+Maxime Bouton
 Michael Tautschnig
 Michael Weber
 Mikuláš Klokočka

spot/misc/tmpfile.cc

@@ -20,7 +20,7 @@
 #include "config.h"
 #include <spot/misc/tmpfile.hh>
 #include <errno.h>
-#include <cstdlib>
+#include <stdlib.h>
 #include <unistd.h>
 #include <string.h>