doc/examples/http_multiclient: fix compilation

Fixes error: no previous prototype for ‘process_client’ [-Werror=missing-prototypes]

doc/examples/http_multiclient.c

@@ -33,7 +33,7 @@
 #include <libavutil/opt.h>
 #include <unistd.h>
 
-void process_client(AVIOContext *client, const char *in_uri)
+static void process_client(AVIOContext *client, const char *in_uri)
 {
     AVIOContext *input = NULL;
     uint8_t buf[1024];