i965g: Fix scons build of winsys

src/gallium/winsys/i965/drm/SConscript

@@ -2,6 +2,8 @@ Import('*')
 
 env = env.Clone()
 
+env.ParseConfig('pkg-config --cflags libdrm')
+
 i965drm_sources = [
     'i965_drm_api.c',
     'i965_drm_buffer.c',