Remove const from ref_count.ref_count

According to Tom Hacohen this was breaking build with some compilers.

In file included from hb-buffer-private.hh:35:0,
                 from hb-ot-map-private.hh:32,
                 from hb-ot-shape-private.hh:32,
                 from hb-ot-shape.cc:29:
hb-object-private.hh: In constructor '_hb_object_header_t::_hb_object_header_t()':
hb-object-private.hh:97:8: error: uninitialized const member in 'struct hb_reference_count_t'
hb-object-private.hh:51:25: note: 'hb_reference_count_t::ref_count' should be initialized
In file included from hb-ot-shape.cc:33:0:
hb-set-private.hh: In constructor '_hb_set_t::_hb_set_t()':
hb-set-private.hh:37:8: note: synthesized method '_hb_object_header_t::_hb_object_header_t()' first required here
hb-ot-shape.cc: In function 'void hb_ot_shape_glyphs_closure(hb_font_t*, hb_buffer_t*, const hb_feature_t*, unsigned int, hb_set_t*)':
hb-ot-shape.cc:521:12: note: synthesized method '_hb_set_t::_hb_set_t()' first required here

src/hb-object-private.hh

@@ -49,7 +49,7 @@
 
 struct hb_reference_count_t
 {
-  const hb_atomic_int_t ref_count;
+  hb_atomic_int_t ref_count;
 
 #define HB_REFERENCE_COUNT_INVALID_VALUE ((hb_atomic_int_t) -1)
 #define HB_REFERENCE_COUNT_INVALID {HB_REFERENCE_COUNT_INVALID_VALUE}