libtextstyle: Fix build failure on Cygwin (regression from 2019-03-15).
* libtextstyle/lib/Makefile.am ($(srcdir)/libtextstyle.sym.in): Renamed
from $(srcdir)/libtextstyle.sym.
(libtextstyle.sym): New rule.
(MOSTLYCLEANFILES, MAINTAINERCLEANFILES, EXTRA_DIST): Update accordingly.
(config.h): Depend on libtextstyle.sym in the build directory.
(libtextstyle_la_LDFLAGS: Use libtextstyle.sym in the build directory.