[meta] Fix unused-function-template warning

src/hb-meta.hh

diff --git a/src/hb-meta.hh b/src/hb-meta.hh
index fd6d60f..0a711e1 100644
--- a/src/hb-meta.hh
+++ b/src/hb-meta.hh
@@ -35,7 +35,7 @@
  */
 
 
-template <typename T> static T hb_declval ();
+template <typename T> static inline T hb_declval ();
 
 
 /* Void!  For when we need a expression-type of void. */