stdlib.h is required for free()

--- a/lib/output.c
+++ b/lib/output.c
@@ -20,6 +20,7 @@
 #include <sys/param.h>
 
 #ifdef STDC_HEADERS
+# include <stdlib.h>
 # include <string.h>
 #endif
 
--- a/lib/wrapper.c
+++ b/lib/wrapper.c
@@ -18,6 +18,7 @@
 #include <errno.h>
 
 #ifdef STDC_HEADERS
+# include <stdlib.h>
 # include <string.h>
 #endif