util/strndup: move header inclusion as applicable

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Vedran Miletić <vedran@miletic.net>
Acked-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>

src/util/strndup.h

@@ -24,10 +24,9 @@
 #ifndef STRNDUP_H
 #define STRNDUP_H
 
-#include <stdlib.h> // size_t
-
 #if defined(_WIN32)
 
+#include <stdlib.h> // size_t
 #include <string.h>
 
 #ifdef __cplusplus