xgettext: In language Vala, fix a format string handling bug.
* gettext-tools/src/xgettext.c (xgettext_record_flag): Store format_c related
flags also in flag_table_vala.
* gettext-tools/tests/xgettext-vala-5: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
