build: Fix build failure with a preinstalled libxml (regression from 2019-04-02).
* gettext-tools/src/Makefile.am (AM_CPPFLAGS): Remove -I options that point into
libtextstyle/lib.
(textstyle.h): New rule.
(BUILT_SOURCES, MOSTLYCLEANFILES): Add textstyle.h and textstyle/*.h.
