meson: Fix define for USE_SSE41

Before we were adding -DHAVE_SSE41 which isn't what the code is
looking for, so some uses of the sse4.1 code were always being
skipped.

v2: Don't add any compile check for the quite old -msse4.1 option (Dylan)

Fixes: 84486f6462 ("meson: Enable SSE4.1 optimizations")
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
diff --git a/meson.build b/meson.build
index 97619f7..eb85a4a 100644
--- a/meson.build
+++ b/meson.build
@@ -772,7 +772,7 @@
 endforeach
 
 if host_machine.cpu_family().startswith('x86')
-  pre_args += '-DHAVE_SSE41'
+  pre_args += '-DUSE_SSE41'
   with_sse41 = true
   sse41_args = ['-msse4.1']