install-info: Add missing stdlib.h

Fix build failure, getenv is declared in stdlib.h.

utils/install-info.c

@@ -22,6 +22,7 @@
 #include <errno.h>
 #include <string.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 #define WARN "install-info: warning: "
 #define FAIL "install-info: failure: "