swr: use the correct variable for no undefined symbols

swr: use the correct variable for no undefined symbols

The variable name was missing a leading LD_, which resulted in a missing
check for unresolved symbols in the backend binaries.

With the link addressed with earlier patches, we can correct the typo.

Thanks to Laurent for the help spotting this.

v2: Split from a larger patch.

Cc: mesa-stable@lists.freedesktop.org
Cc: Bruce Cherniak <bruce.cherniak@intel.com>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Cc: Laurent Carlier <lordheavym@gmail.com>
Fixes: 9475251145174882b532 "swr: standardize linkage and check for
                             unresolved symbols"
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reported-by: Laurent Carlier <lordheavym@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>