gtk_css_provider_load_from_data: element-type=guint8 does not match const char *

There is a mismath between the (element-type guint8) annotation in the documentation comment and the C type const char * for parameter data of gtk_css_provider_load_from_data. This causes a warning

warning: passing 'const guchar *' (aka 'const unsigned char *') to parameter of type 'const char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]

in the C glue code I generate for the OCaml bindings for this API.