automake: add SWR to `make distcheck' gallium drivers

Will allows us to catch missing files and build issues before getting
the tarball out for general consumption.

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/Makefile.am b/Makefile.am
index 4148eea..b0fbed6 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -43,7 +43,7 @@
 	--disable-llvm-shared-libs \
 	--with-egl-platforms=x11,wayland,drm,surfaceless \
 	--with-dri-drivers=i915,i965,nouveau,radeon,r200,swrast \
-	--with-gallium-drivers=i915,ilo,nouveau,r300,r600,radeonsi,freedreno,svga,swrast,vc4,virgl \
+	--with-gallium-drivers=i915,ilo,nouveau,r300,r600,radeonsi,freedreno,svga,swrast,vc4,virgl,swr \
 	--with-vulkan-drivers=intel
 
 ACLOCAL_AMFLAGS = -I m4