glsl: return cloned signature, not the builtin one

glsl: return cloned signature, not the builtin one

The builtin data can get released with a glReleaseShaderCompiler call.
We're careful everywhere to clone everything that comes out of builtins
except here, where we accidentally return the signature belonging to the
builtin version, rather than the locally-cloned one.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Tested-by: Rob Herring <robh@kernel.org>
Cc: mesa-stable@lists.freedesktop.org