Add hb_tag_from_string()

src/hb-common.h

diff --git a/src/hb-common.h b/src/hb-common.h
index 594d358..74d8512 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -65,6 +65,9 @@
 				((const char *) s)[3]))
 #define HB_TAG_NONE HB_TAG(0,0,0,0)
 
+hb_tag_t hb_tag_from_string (const char *s);
+
+
 typedef uint32_t hb_codepoint_t;
 typedef int32_t hb_position_t;
 typedef uint32_t hb_mask_t;