doc/requirements.txt: Add standard-imghdr

Trying to run older sphinx 5.3 on python 3.13 requires to use imghdr,
which was removed from standard library. Update it using pip.

Signed-off-by: Petr Vorel <pvorel@suse.cz>

Author: pevik

doc/requirements.txt

@@ -2,6 +2,6 @@
 # sphinx-rtd-theme is compatible with sphinx.
 sphinx==5.3.0
 sphinx-rtd-theme==2.0.0
-
+standard-imghdr
 linuxdoc==20231020
 sphinxcontrib-spelling==7.7.0