Try fixing build with Sun Studio

Trying to fix:
https://circleci.com/gh/harfbuzz/harfbuzz/6635

Also part of:
https://github.com/harfbuzz/harfbuzz/issues/630

src/hb-private.hh

@@ -120,7 +120,9 @@ extern "C" void  hb_free_impl(void *ptr);
 #endif
 
 #ifndef HB_INTERNAL
-# if !defined(__MINGW32__) && !defined(__CYGWIN__)
+# ifdef __SUNPRO_CC
+#  define HB_INTERNAL __hidden
+# elif !defined(__MINGW32__) && !defined(__CYGWIN__)
 #  define HB_INTERNAL __attribute__((__visibility__("hidden")))
 # else
 #  define HB_INTERNAL
@@ -136,6 +138,11 @@ extern "C" void  hb_free_impl(void *ptr);
 #define HB_FUNC __func__
 #endif
 
+#ifdef __SUNPRO_CC
+/* https://github.com/harfbuzz/harfbuzz/issues/630 */
+#define __restrict
+#endif
+
 /*
  * Borrowed from https://bugzilla.mozilla.org/show_bug.cgi?id=1215411
  * HB_FALLTHROUGH is an annotation to suppress compiler warnings about switch