configure: Fix the broken check for libfdt
configure: Fix the broken check for libfdt

If success action is specified to AC_CHECK_LIB, then default action of
updating ${LIBS} variable is not skipped.

Signed-off-by: Amitay Isaacs <a505e7c038746dbeb3edcd8d146dbd515432801f@ozlabs.org>

open-power/pdbg,open-power/pdbg,open-power/pdbg