Fix compiler warnings for gobject introspection

[brawer]

Compiling current harfbuzz head with --with-gobject --enable-introspection gives a bunch of warnings:

$ sudo apt-get install libgirepository1.0-dev
$ ./configure --with-gobject --enable-introspection

hb-buffer.cc:667: Warning: HarfBuzz: unknown annotation: xconstructor
hb-face.cc:152: Warning: HarfBuzz: unknown annotation: xconstructor
hb-font.cc:402: Warning: HarfBuzz: unknown annotation: xconstructor
hb-font.cc:1113: Warning: HarfBuzz: unknown annotation: xconstructor
hb-set.cc:34: Warning: HarfBuzz: unknown annotation: xconstructor
hb-shape-plan.cc:98: Warning: HarfBuzz: unknown annotation: xconstructor
hb-unicode.cc:154: Warning: HarfBuzz: unknown annotation: xconstructor
hb-unicode.h:452: Warning: HarfBuzz: multiple comment blocks documenting 'hb_unicode_compose:' identifier (already seen at hb-unicode.cc:387).
hb-unicode.h:463: Warning: HarfBuzz: multiple comment blocks documenting 'hb_unicode_decompose:' identifier (already seen at hb-unicode.cc:409).
hb-unicode.h:474: Warning: HarfBuzz: multiple comment blocks documenting 'hb_unicode_decompose_compatibility:' identifier (already seen at hb-unicode.cc:431).
hb-gobject-structs.h:44: Error: HarfBuzz: identifier not found on the first line:
 * Since: 0.9.2
   ^
hb-gobject-structs.h:50: Error: HarfBuzz: identifier not found on the first line:
 * Since: 0.9.2
   ^
hb-gobject-structs.h:56: Error: HarfBuzz: identifier not found on the first line:
 * Since: 0.9.2
   ^
hb-gobject-structs.h:62: Error: HarfBuzz: identifier not found on the first line:
 * Since: 0.9.2
   ^
hb-gobject-structs.h:68: Error: HarfBuzz: identifier not found on the first line:
 * Since: 0.9.2
   ^
hb-gobject-structs.h:80: Error: HarfBuzz: identifier not found on the first line:
 * Since: 0.9.2
   ^
hb-buffer.cc:55: Warning: HarfBuzz: invalid "transfer" annotation for gint: only valid for array, struct, union, boxed, object and interface types
hb-shape.cc:397: Warning: HarfBuzz: hb_shape: invalid return annotation
hb-ot-color.cc:152: Warning: HarfBuzz: invalid "optional" annotation: only valid for out parameters
hb-ft.cc:481: Warning: HarfBuzz: hb_ft_face_create: argument ft_face: Unresolved type: u'FT_Face'
hb-ft.cc:538: Warning: HarfBuzz: hb_ft_face_create_cached: argument ft_face: Unresolved type: u'FT_Face'
hb-ft.cc:516: Warning: HarfBuzz: hb_ft_face_create_referenced: argument ft_face: Unresolved type: u'FT_Face'
hb-ft.cc:563: Warning: HarfBuzz: hb_ft_font_create: argument ft_face: Unresolved type: u'FT_Face'
hb-ft.cc:596: Warning: HarfBuzz: hb_ft_font_create_referenced: argument ft_face: Unresolved type: u'FT_Face'
hb-ft.h:111: Warning: HarfBuzz: hb_ft_font_get_face: return value: Unresolved type: u'FT_Face'

[behdad]

Fixed bunch of them.