Add missing include guards

In practice, these headers are included only once, but it's a good
practice to always use include guards.

app/src/server.h

@@ -1,3 +1,6 @@
+#ifndef SERVER_H
+#define SERVER_H
+
 #include "command.h"
 
 process_t push_server(const char *serial);
@@ -6,3 +9,5 @@ process_t disable_tunnel(const char *serial);
 
 process_t start_server(const char *serial, Uint16 max_size, Uint32 bit_rate);
 void stop_server(process_t server);
+
+#endif