configure: Fix build with XFree

The build is broken on ppc64-linux, possibly only with new binutils:

ld: hw/lm32/../milkymist-tmu2.o: undefined reference to symbol 'XFree'
ld: note: 'XFree' is defined in DSO /lib64/libX11.so.6 so try \
  adding it to the linker command line

So let's follow the linker's advice.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -2388,7 +2388,7 @@ fi
 ##########################################
 # opengl probe, used by milkymist-tmu2
 if test "$opengl" != "no" ; then
-  opengl_libs="-lGL"
+  opengl_libs="-lGL -lX11"
   cat > $TMPC << EOF
 #include <X11/Xlib.h>
 #include <GL/gl.h>