scons: Add main/pipelineobj.c to src/mesa/SConscript.

This patch fixes this SCons build error.

build/linux-x86_64-debug/mesa/libmesa.a(context.os): In function `init_attrib_groups':
src/mesa/main/context.c:815: undefined reference to `_mesa_init_pipeline'

Signed-off-by: Vinson Lee <vlee@freedesktop.org>

src/mesa/SConscript

@@ -98,6 +98,7 @@ main_sources = [
     'main/pack.c',
     'main/pbo.c',
     'main/performance_monitor.c',
+    'main/pipelineobj.c',
     'main/pixel.c',
     'main/pixelstore.c',
     'main/pixeltransfer.c',