MINOR: hlua: declare hlua_gethlua() function

Declaring hlua_gethlua() function to make it usable from hlua_fcn.c.
diff --git a/src/hlua.c b/src/hlua.c
index 2c68d4f..98ee441 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -1294,7 +1294,7 @@
  *  - hlua_gethlua : return the hlua context associated with an lua_State.
  *  - hlua_sethlua : create the association between hlua context and lua_state.
  */
-static inline struct hlua *hlua_gethlua(lua_State *L)
+inline struct hlua *hlua_gethlua(lua_State *L)
 {
 	struct hlua **hlua = lua_getextraspace(L);
 	return *hlua;