Add missing includes

Include these headers explicitly instead of relying on transitivity.

app/src/sys/unix/file.c

@@ -2,6 +2,7 @@
 
 #include <limits.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <unistd.h>

app/src/v4l2_sink.c

@@ -1,5 +1,7 @@
 #include "v4l2_sink.h"
 
+#include <string.h>
+
 #include "util/log.h"
 #include "util/str.h"
 

app/tests/test_str.c

@@ -3,6 +3,7 @@
 #include <assert.h>
 #include <limits.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include "util/str.h"

app/tests/test_strbuf.c

@@ -2,6 +2,7 @@
 
 #include <assert.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include "util/strbuf.h"