build: add history.c to the list of files with translatable strings

The recent migration of the history routines forgot to add the new
file to the list of files from which strings should be extracted.

This fixes https://savannah.gnu.org/bugs/?52122.

po/POTFILES.in

@@ -5,6 +5,7 @@ src/color.c
 src/files.c
 src/global.c
 src/help.c
+src/history.c
 src/nano.c
 src/prompt.c
 src/rcfile.c