git-gui: sync Makefiles with git.git

In git.git, commit 5309c1e9fb (Makefile: set default goals in
makefiles, 2025-02-15) touched two Makefiles in the git-git/ directory.
Import these changes, so that the trees can converge again with the
next merge of this repository into git.git.

Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Makefile

@@ -1,3 +1,4 @@
+# The default target of this Makefile is...
 all::
 
 # Define V=1 to have a more verbose compile.

po/glossary/Makefile

@@ -1,3 +1,6 @@
+# The default target of this Makefile is...
+update-po::
+
 PO_TEMPLATE = git-gui-glossary.pot
 
 ALL_POFILES = $(wildcard *.po)