meson: fix swr KNL build

There's a typo in one of the #defines that breaks compilation.

Fixes: e0b037d697
       ("meson: Build SWR driver")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109023
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/gallium/drivers/swr/meson.build

@@ -264,7 +264,7 @@ if with_swr_arches.contains('knl')
     [files_swr_common, files_swr_arch],
     cpp_args : [
       swr_cpp_args, swr_knl_args, '-DKNOB_ARCH=KNOB_ARCH_AVX512',
-      '-DKNOB_ARCH_KNIGHTS',
+      '-DSIMD_ARCH_KNIGHTS',
     ],
     link_args : [ld_args_gc_sections],
     include_directories : [swr_incs],