girepository: Add missing includes of corresponding headers

Make sure that each C file in `girepository/` includes its corresponding
header file (if such a file exists).

This ensures that the first declaration the compiler sees for each
function contains the symbol visibility macros. This fixes symbol export
(`dllexport`) on Windows platforms.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Helps: #3155

girepository/gistructinfo.c

@@ -31,6 +31,7 @@
 #include <girepository/girepository.h>
 #include "girepository-private.h"
 #include "gitypelib-internal.h"
+#include "gistructinfo.h"
 
 /**
  * SECTION:gistructinfo