Remove unused headers from uu_string.c

The malloc.h include is gratuitous and runs in to the following error
on FreeBSD:

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9509

lib/libuutil/uu_string.c

@@ -29,8 +29,6 @@
 
 #include <string.h>
 #include <sys/types.h>
-#include <stdio.h>
-#include <malloc.h>
 #include <ctype.h>
 #include "libuutil.h"