load-tester: Include string.h for strcmp() on some platforms

src/libcharon/plugins/load_tester/load_tester.c

@@ -21,6 +21,7 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <errno.h>
 
 /**