windows: fix dll builds

WebPSafe* need to be marked external to allow mux/demux to access them
through libwebp.dll

Change-Id: Ib6620e00d376f7aa5a0550e1e244f759977f97a0

src/utils/utils.h

@@ -35,13 +35,13 @@ extern "C" {
 // somewhere (like: malloc(num_pixels * sizeof(*something))). That's why this
 // safe malloc() borrows the signature from calloc(), pointing at the dangerous
 // underlying multiply involved.
-void* WebPSafeMalloc(uint64_t nmemb, size_t size);
+WEBP_EXTERN(void*) WebPSafeMalloc(uint64_t nmemb, size_t size);
 // Note that WebPSafeCalloc() expects the second argument type to be 'size_t'
 // in order to favor the "calloc(num_foo, sizeof(foo))" pattern.
-void* WebPSafeCalloc(uint64_t nmemb, size_t size);
+WEBP_EXTERN(void*) WebPSafeCalloc(uint64_t nmemb, size_t size);
 
 // Companion deallocation function to the above allocations.
-void WebPSafeFree(void* const ptr);
+WEBP_EXTERN(void) WebPSafeFree(void* const ptr);
 
 //------------------------------------------------------------------------------
 // Reading/writing data.