fix shared object build with -fvisibility=hidden

set WEBP_EXTERN to visibility=default
+ explicitly mark VP8GetCPUInfo as it's referenced within the examples

Change-Id: Ie3d2b15088e888f0b55203b205993eba75899d99

src/dsp/dsp.h

@@ -100,7 +100,7 @@ typedef enum {
 } CPUFeature;
 // returns true if the CPU supports the feature.
 typedef int (*VP8CPUInfo)(CPUFeature feature);
-extern VP8CPUInfo VP8GetCPUInfo;
+WEBP_EXTERN(VP8CPUInfo) VP8GetCPUInfo;
 
 //------------------------------------------------------------------------------
 // Encoding