tools_common: fix build w/encoders/decoders disabled

gcc-based builds will allow a 0-element array, but visual studio builds
will not; this change hides the encoder and decoder specific symbols as
modules using them are selected based on the configuration.

Change-Id: Ic16ba9d12241070ec689dc5880164c14a4f7ca44

tools_common.c

diff --git a/tools_common.c b/tools_common.c
index 8d356af..fe3401c 100644
--- a/tools_common.c
+++ b/tools_common.c
@@ -130,6 +130,8 @@
   return shortread;
 }
 
+#if CONFIG_ENCODERS
+
 static const VpxInterface vpx_encoders[] = {
 #if CONFIG_VP8_ENCODER
   {"vp8", VP8_FOURCC, &vpx_codec_vp8_cx},
@@ -160,6 +162,10 @@
   return NULL;
 }
 
+#endif  // CONFIG_ENCODERS
+
+#if CONFIG_DECODERS
+
 static const VpxInterface vpx_decoders[] = {
 #if CONFIG_VP8_DECODER
   {"vp8", VP8_FOURCC, &vpx_codec_vp8_dx},
@@ -202,6 +208,8 @@
   return NULL;
 }
 
+#endif  // CONFIG_DECODERS
+
 // TODO(dkovalev): move this function to vpx_image.{c, h}, so it will be part
 // of vpx_image_t support
 int vpx_img_plane_width(const vpx_image_t *img, int plane) {