Update SConscript

Do not include the example when not selected.

SConscript

@@ -3,7 +3,8 @@ from building import *
 cwd = GetCurrentDir()
 path = [cwd + '/inc']
 src  = Glob('src/*.c')
-src += Glob('samples/ppp_sample.c')
+if GetDepend(['PPP_DEVICE_SAMPLE']):
+	src += Glob('samples/ppp_sample.c')
 
 # Air720
 if GetDepend(['PPP_DEVICE_USING_AIR720']):