minidlna: exclude "po" directory to fix CONFIG_BUILD_NLS=y builds

minidlna: exclude "po" directory to fix CONFIG_BUILD_NLS=y builds

This fixes:
*** error: gettext infrastructure mismatch: using a Makefile.in.in from gettext version 0.18 but the autoconf macros are from gettext version 0.19

Makefile of minidlna package specifies PKG_FIXUP:=autoreconf. That
results in calling autoreconf with multiple arguments, including many -I
ones. One of autoreconf steps is calling aclocal with the same set of -I
arguments.

All of that results in:
1) aclocal using staging_dir's /usr/share/aclocal and its po.m4
2) not using minidlna's po.m4
3) not updating Makefile.in.in

If staging_dir's po.m4 has different GETTEXT_MACRO_VERSION than the
minidlna's one it'll result in a mismatch in the Makefile.in. Ideally we
should take care of regenerating Makefile.in.in but this isn't
currentlly supported. As localization isn't properly supported anyway
(no shipping .mo files) it's safe to just disable building po files.

Added patch comes from the master branch commit d5fcc972ba57d
("multimedia/minidlna: Update to 1.2.0").

Fixes: 72928442614d9 ("minidlna: backport fixes from 1.1.6 and 1.2.0 releases")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>