build: Fix build failure when people set DESTDIR already during 'make'.
Reported by Henrik Holst <henrik.holst@outlook.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2024-09/msg00005.html>.
* gettext-tools/src/Makefile.am (textstyle.h): Pass an empty DESTDIR to
libtextstyle/lib/Makefile.
