gallivm: Fix a signature mismatch warning

gallivm: Fix a signature mismatch warning

gcc 11 says:

../src/gallium/auxiliary/gallivm/lp_bld_format_soa.c:84:49: warning: argument 3 of type ‘struct LLVMOpaqueValue * const*’ declared as a pointer [-Warray-parameter=]
   84 |                             const LLVMValueRef *unswizzled,
      |                             ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
In file included from ../src/gallium/auxiliary/gallivm/lp_bld_format_soa.c:42:
../src/gallium/auxiliary/gallivm/lp_bld_format.h:126:48: note: previously declared as an array ‘struct LLVMOpaqueValue * const[4]’
  126 |                             const LLVMValueRef unswizzled[4],
      |                             ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10738>