gio/test/resources.c: Fix for Windows

We need to use g_content_type_get_mime_type() to look up the mime type of
the file from the registry on the content type that was acquired on
Windows, as g_file_info_get_content_type() does not acquire the
file mime type (unlike on *NIX).

g_content_type_get_mime_type() on *NIX is more or less an no-op as it
simply returns the g_strdup()-ed version of the passed-in content type.

This will enable the resources test to pass on Windows.

https://bugzilla.gnome.org/show_bug.cgi?id=711047
tests/libmoduletestplugins*.c: Update Prototypes

Visual C++ does not like function declarations being different from
their prototypes, so make the prototypes match the declarations by
decorating them with G_MODULE_EXPORT.

https://bugzilla.gnome.org/show_bug.cgi?id=711047