configure: Document --enable-vdpau.

The option is disabled by default, so enabling it should be documented.

configure

@@ -107,7 +107,7 @@ Configuration options:
   --disable-mdct           disable MDCT code
   --disable-rdft           disable RDFT code
   --enable-vaapi           enable VAAPI code
-  --disable-vdpau          disable VDPAU code
+  --enable-vdpau           enable VDPAU code
   --disable-dxva2          disable DXVA2 code
   --enable-runtime-cpudetect detect cpu capabilities at runtime (bigger binary)
   --enable-hardcoded-tables use hardcoded tables instead of runtime generation