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-coretext.cc

diff --git a/src/hb-coretext.cc b/src/hb-coretext.cc
index 4923561..66f0fce 100644
--- a/src/hb-coretext.cc
+++ b/src/hb-coretext.cc
@@ -410,7 +410,7 @@
   feature_record_t rec;
   unsigned int order;
 
-  static int cmp (const void *pa, const void *pb) {
+  HB_INTERNAL static int cmp (const void *pa, const void *pb) {
     const active_feature_t *a = (const active_feature_t *) pa;
     const active_feature_t *b = (const active_feature_t *) pb;
     return a->rec.feature < b->rec.feature ? -1 : a->rec.feature > b->rec.feature ? 1 :
@@ -428,7 +428,7 @@
   bool start;
   active_feature_t feature;
 
-  static int cmp (const void *pa, const void *pb) {
+  HB_INTERNAL static int cmp (const void *pa, const void *pb) {
     const feature_event_t *a = (const feature_event_t *) pa;
     const feature_event_t *b = (const feature_event_t *) pb;
     return a->index < b->index ? -1 : a->index > b->index ? 1 :