autotools: copy also remove-potcdate.sin from ${STAGING_DATADIR_NATIVE}/gettext/po

autotools: copy also remove-potcdate.sin from ${STAGING_DATADIR_NATIVE}/gettext/po

* some packages (e.g. uptime, cpu, forecasts, news from meta-efl)
  don't have remove-potcdate.sin in po subdirectory, but Makefile.in.in
  supplied by autotools.bbclass depends on it and fails without like this:
  | make[3]: Entering directory `/OE/shr-core/tmp-eglibc/work/x86_64-oe-linux/uptime/0.0.2+svnr80477-r0.0/E-MODULES-EXTRA/uptime/po'
  | make[3]: *** No rule to make target `remove-potcdate.sin', needed by `remove-potcdate.sed'.  Stop.
  | make[3]: Leaving directory `/OE/shr-core/tmp-eglibc/work/x86_64-oe-linux/uptime/0.0.2+svnr80477-r0.0/E-MODULES-EXTRA/uptime/po'
  | make[2]: *** [uptime.pot] Error 2

(From OE-Core rev: 5a3116638394dc72c54cf5d4f8bd42a7be0b1768)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>