wrlib: moved 'RShutdown' from 'load.c' to 'misc.c' for consistency

The cleanup function is actually not specific to loading images and could
do a lot more, so it is probably better placed in 'misc.c'

wrlib/imgformat.h

@@ -75,6 +75,8 @@ RImage *RLoadWEBP(const char *file);
 
 #ifdef USE_MAGICK
 RImage *RLoadMagick(const char *file_name);
+
+void RReleaseMagick(void);
 #endif
 
 /*