Add license information for the bullhorn icon.

blank-canvas/style.css

@@ -55,6 +55,11 @@ License: Apache License 2.0
 https://github.com/googlefonts/noto-emoji/blob/master/svg/emoji_u1f354.svg
 Modified and used as a block pattern image.
 
+Bullhorn Icon by the Consumer Financial Protection Bureau, US
+License: Public Domain
+https://thenounproject.com/term/bullhorn/89665/
+Modified and used as a block pattern image.
+
 Self-Portrait with Two Pupils, Marie Gabrielle Capet and Marie Marguerite Carreaux de Rosemond
 by Adélaïde Labille-Guiard, 1785
 License: Public Domain