libprelude: workaround build failures with libtool 2.4

libprelude: workaround build failures with libtool 2.4

Like:

In file included from prelude-plugin.c:56:0:
/include/prelude-plugin.h:66:20: error: conflicting types for 'lt__PROGRAM__LTX_preloaded_symbols'
/oe/build-angstrom-next/angstrom-dev/sysroots/armv7a-angstrom-linux-gnueabi/usr/include/ltdl.h:106:36: note: previous declaration of 'lt__PROGRAM__LTX_preloaded_symbols' was here
make[4]: *** [prelude-plugin.lo] Error 1

Originally reported by Paul Menzel <paulepanter@users.sourceforge.net>.

Forwarded upstream as https://dev.prelude-technologies.com/issues/386

Signed-off-by: Roman I Khimov <khimov@altell.ru>
Acked-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>