Hide a few static methods

Looks like static methods that do not get inlined end up exported.
We have a lot more.  Need to protect all at some point.  Wish there
was an easier way, like the visibility flag we pass that automatically
hides all inline methods.

Was exposed by check-symbols.sh when compiling on OS X 10.14 with:

$ make CPPFLAGS=-Oz CXXFLAGS=-flto=thin LDFLAGS=-lc++

src/hb-ot-layout-gpos-table.hh

diff --git a/src/hb-ot-layout-gpos-table.hh b/src/hb-ot-layout-gpos-table.hh
index 795853a..a7257df 100644
--- a/src/hb-ot-layout-gpos-table.hh
+++ b/src/hb-ot-layout-gpos-table.hh
@@ -173,15 +173,15 @@
     return true;
   }
 
-  static OffsetTo<Device>& get_device (Value* value)
+  HB_INTERNAL static OffsetTo<Device>& get_device (Value* value)
   { return *CastP<OffsetTo<Device> > (value); }
-  static const OffsetTo<Device>& get_device (const Value* value, bool *worked=nullptr)
+  HB_INTERNAL static const OffsetTo<Device>& get_device (const Value* value, bool *worked=nullptr)
   {
     if (worked) *worked |= bool (*value);
     return *CastP<OffsetTo<Device> > (value);
   }
 
-  static const HBINT16& get_short (const Value* value, bool *worked=nullptr)
+  HB_INTERNAL static const HBINT16& get_short (const Value* value, bool *worked=nullptr)
   {
     if (worked) *worked |= bool (*value);
     return *CastP<HBINT16> (value);
@@ -1576,7 +1576,7 @@
     dispatch (&c);
   }
 
-  static bool apply_recurse_func (hb_ot_apply_context_t *c, unsigned int lookup_index);
+  HB_INTERNAL static bool apply_recurse_func (hb_ot_apply_context_t *c, unsigned int lookup_index);
 
   template <typename context_t>
   static typename context_t::return_t dispatch_recurse_func (context_t *c, unsigned int lookup_index);