Fix API docs build without libexif

origin commit:
https://gitlab.gnome.org/GNOME/eog/commit/d45b7f1

Author: fxri

doc/reference/Makefile.am

@@ -55,12 +55,10 @@ IGNORE_HFILES= \
 	eom-enum-types.h
 
 if !HAVE_EXIF
-if !HAVE_EXEMPI
 IGNORE_HFILES +=			\
 	eom-exif-util.h			\
 	eom-metadata-details.h
 endif
-endif
 
 # Images to copy into HTML directory.
 # e.g. HTML_IMAGES=$(top_srcdir)/gtk/stock-icons/stock_about_24.png