intl: Fix build failure with "make -j".
Reported by Christian Weisgerber <naddy@mips.inka.de> at
<https://lists.gnu.org/archive/html/bug-gettext/2023-09/msg00005.html>.
* gettext-runtime/intl/Makefile.am (langprefs.lo, log.lo): Depend on gettextP.h
and its subordinate includes.
