configure: correctly report if we're building the sw/xlib winsys

While looking at bug 75356, I've noticed that the presence of
x11 egl platform pulls in sw/xlib as "needed" but fails to
report so at the end of configure.

Tested-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

configure.ac

@@ -1448,6 +1448,7 @@ else
 fi
 
 if echo "$egl_platforms" | grep 'x11' >/dev/null 2>&1; then
+    GALLIUM_WINSYS_DIRS="$GALLIUM_WINSYS_DIRS sw/xlib"
     NEED_WINSYS_XLIB=yes
 fi
 AM_CONDITIONAL(HAVE_EGL_PLATFORM_X11, echo "$egl_platforms" | grep 'x11' >/dev/null 2>&1)