qrexec-lib: fix compile warnings

qrexec-lib/copy-file.c

@@ -12,7 +12,7 @@ int copy_file(int outfd, int infd, long long size, unsigned long *crc32)
 	int ret;
 	int count;
 	while (written < size) {
-		if (size - written > sizeof(buf))
+		if (size - written > (int)sizeof(buf))
 			count = sizeof buf;
 		else
 			count = size - written;

qrexec-lib/txrx-vchan.c

@@ -29,7 +29,7 @@
 
 static struct libvchan *ctrl;
 static int is_server;
-int write_all_vchan_ext(void *buf, int size)
+int write_all_vchan_ext(const void *buf, int size)
 {
 	int written = 0;
 	int ret;