enum: fix FTBFS with gi-docgen 2023.3

Debian project detects FTBFS in libhinawa 4.0.0 package with
gi-docgen 2023.3. This commit fixes it.

Reference: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1058535
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>