Refactor: Rename locating-rule.[hc].
* gettext-tools/src/locating-rules.h: Renamed from gettext-tools/src/locating-rule.h.
* gettext-tools/src/locating-rules.c: Renamed from gettext-tools/src/locating-rule.c.
* gettext-tools/src/FILES: Update.
* gettext-tools/src/Makefile.am (noinst_HEADERS, libgettextsrc_la_SOURCES): Update.
* gettext-tools/src/msgfmt.c: Update.
* gettext-tools/src/xgettext.c: Update.
* gettext-tools/po/POTFILES.in: Update.
