build: enable st/va with nouveau driver

build: enable st/va with nouveau driver

vainfo fails in vaDriverInit because "dd_create_screen"
does not reach strcmp(driver_name, "nouveau") code.
Indeed when compiling the va target.c, the macro GALLIUM_NOUVEAU
is not defined.
This patch define the macro the same it is done for dri and
vdpau targets.

Tested with:
./autogen.sh --enable-glx --enable-gles2 --enable-egl --enable-vdpau --enable-glx-tls=yes --enable-va
--with-gallium-drivers=swrast,nouveau --with-dri-drivers=swrast,nouveau --with-egl-platforms=x11

LIBVA_DRIVER_NAME=gallium vainfo
Output:
vainfo: Driver version: mesa gallium vaapi
vainfo: Supported profile and entrypoints
VAProfileMPEG2Simple                  :	VAEntrypointVLD
      VAProfileMPEG2Main              :	VAEntrypointVLD
      VAProfileMPEG4Simple            :	VAEntrypointVLD
      VAProfileMPEG4AdvancedSimple    :	VAEntrypointVLD
      VAProfileVC1Simple              :	VAEntrypointVLD
      VAProfileVC1Main                :	VAEntrypointVLD
      VAProfileVC1Advanced            :	VAEntrypointVLD
      VAProfileH264Baseline           :	VAEntrypointVLD
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileNone                   :	VAEntrypointVideoProc

Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>