Fixed a bug which caused multiply defined symbol errors during linking

if the PNG saving routine was #included.

dlib/image_saver/save_png.h

@@ -21,11 +21,11 @@ namespace dlib
 #ifdef DLIB_PNG_SUPPORT
     // Don't do anything when libpng calls us to tell us about an error.  Just return to 
     // our own code and throw an exception (at the long jump target).
-    void png_reader_user_error_fn_silent(png_structp  png_struct, png_const_charp ) 
+    inline void png_reader_user_error_fn_silent(png_structp  png_struct, png_const_charp ) 
     {
         longjmp(png_jmpbuf(png_struct),1);
     }
-    void png_reader_user_warning_fn_silent(png_structp , png_const_charp ) 
+    inline void png_reader_user_warning_fn_silent(png_structp , png_const_charp ) 
     {
     }
 #endif