configure: remove trailing "-a" in swr architecture test

Fixes "configure: line 27326: test: argument expected"

CC: mesa-stable@lists.freedesktop.org
Reviewed-by: Matt Turner <mattst88@gmail.com>
diff --git a/configure.ac b/configure.ac
index d2704bc..f131788 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2545,7 +2545,7 @@
             if test "x$HAVE_SWR_AVX" != xyes -a \
                     "x$HAVE_SWR_AVX2" != xyes -a \
                     "x$HAVE_SWR_KNL" != xyes -a \
-                    "x$HAVE_SWR_SKX" != xyes -a; then
+                    "x$HAVE_SWR_SKX" != xyes; then
                AC_MSG_ERROR([swr enabled but no swr architectures selected])
             fi