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/controlevent.h

@@ -1,5 +1,5 @@
-#ifndef CONTROL_EVENT_H
+#ifndef CONTROLEVENT_H
-#define CONTROL_EVENT_H
+#define CONTROLEVENT_H
 
 #include <SDL2/SDL_mutex.h>
 #include <SDL2/SDL_stdinc.h>

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

app/src/strutil.h

@@ -1,3 +1,6 @@
+#ifndef STRUTIL_H
+#define STRUTIL_H
+
 #include <stddef.h>
 
 // like strncpy, except:
@@ -12,3 +15,5 @@ size_t xstrncpy(char *dest, const char *src, size_t n);
 // returns the number of chars actually written (max n-1) if no trucation
 // occurred, or n if truncated
 size_t xstrjoin(char *dst, const char *const tokens[], char sep, size_t n);
+
+#endif

app/src/tinyxpm.h

@@ -1,3 +1,8 @@
+#ifndef TINYXPM_H
+#define TINYXPM_H
+
 #include <SDL2/SDL.h>
 
 SDL_Surface *read_xpm(char *xpm[]);
+
+#endif