targets/xvmc: use the non-inline sw helpers

This was missed in commit 59cfb21d ("targets: use the non-inline sw
helpers").

Fixes build failure:

  CXXLD    libXvMCgallium.la
../../../../src/gallium/auxiliary/pipe-loader/.libs/libpipe_loader_static.a(libpipe_loader_static_la-pipe_loader_sw.o):(.data.rel.ro+0x0): undefined reference to `sw_screen_create'
collect2: error: ld returned 1 exit status
Makefile:756: recipe for target 'libXvMCgallium.la' failed
make[3]: *** [libXvMCgallium.la] Error 1

Trivial.

src/gallium/targets/xvmc/target.c

@@ -1 +1,2 @@
 #include "target-helpers/drm_helper.h"
+#include "target-helpers/sw_helper.h"