doc: add Unicode functions to API description

Add include/charset.h to generated HTML documentation

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/api/index.rst b/doc/api/index.rst
index d484c06..2578abd 100644
--- a/doc/api/index.rst
+++ b/doc/api/index.rst
@@ -9,3 +9,4 @@
    efi
    linker_lists
    serial
+   unicode