Add G_VALUE_INIT

The implementation of GValue is not public or documented.  When
allocated on the stack, initializing a GValue is usually done as
documented with:

GValue value = { 0, };

There is lot code around (including WebKit) that added all the missing
fields, resulting in this ugly and non-obvious:

GValue value = { 0, { { 0 } } };

However, this doesn't play nice with -Wmissing-field-initializers for
example. Thus, G_VALUE_INIT.

http://bugzilla.gnome.org/show_bug.cgi?id=654793
http://bugzilla.gnome.org/show_bug.cgi?id=577231

docs/reference/gobject/gobject-sections.txt

@@ -376,6 +376,7 @@ g_variant_type_get_gtype
 <SECTION>
 <TITLE>Generic values</TITLE>
 <FILE>generic_values</FILE>
+G_VALUE_INIT
 G_VALUE_HOLDS
 G_VALUE_TYPE
 G_VALUE_TYPE_NAME