Update lagging libgcobol/Makefile.am to match libgcobol/Makefile.in.
libgcobol/ChangeLog:
PR cobol/122702
* Makefile.am: Include posix/udf/posix-open.cbl.
libgcobol.cc \
posix/shim/errno.cc \
posix/shim/localtime.cc \
+ posix/shim/open.cc \
posix/shim/stat.cc \
stringbin.cc \
valconv.cc \
posix/udf/posix-exit.cbl \
posix/udf/posix-localtime.cbl \
posix/udf/posix-mkdir.cbl \
+ posix/udf/posix-open.cbl \
posix/udf/posix-stat.cbl \
posix/udf/posix-unlink.cbl \
compat/gnu/lib/CBL_CHECK_FILE_EXIST.cbl \