common: Explicitly include context's own header

Previously we were relying on flatpak-utils-private.h having a circular
dependency on this, but I'm intending to remove that during future
refactoring.

Signed-off-by: Simon McVittie <smcv@collabora.com>

common/flatpak-context.c

@@ -19,6 +19,7 @@
  */
 
 #include "config.h"
+#include "flatpak-context-private.h"
 
 #include <string.h>
 #include <fcntl.h>