Removed exlict keyword to address build problems with VS older than 2015.

src/osgPlugins/tga/ReaderWriterTGA.cpp

@@ -325,7 +325,7 @@ struct SafeArray
     operator T*() { return impl; }
 
     template<typename U>
-    explicit operator U() { return (U)impl; }
+    operator U() { return (U)impl; }
 
 private:
     T* impl;