build: add gnulib modules to the list of files with translatable strings

The error messages from gnulib need to be included into nano's POT file
so that those messages get translated when nano is built on a platform
where the modules that contain the messages get used.

This fixes https://savannah.gnu.org/bugs/?56107.

Bug existed since gnulib support was added in version 2.8.0.
The relevant modules were added by commits 3deec435 and 272345cc.

po/POTFILES.in

@@ -14,3 +14,6 @@ src/search.c
 src/text.c
 src/utils.c
 src/winio.c
+
+lib/getopt.c
+lib/regcomp.c