meson: Always wrap "prefix" option with join_paths() to make Windows …

…happy

Both "\" and "/" separators are allowed on Windows but the problem
is that "\" is not converted to "\\" in this case.
It will result to unknown/unrecognized character escape sequence.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/818>

meson.build

@@ -325,7 +325,7 @@ else
       get_option('libdir'), 'gio/modules'))
     core_conf.set_quoted('GIO_LIBDIR', join_paths(get_option('prefix'),
       get_option('libdir')))
-    core_conf.set_quoted('GIO_PREFIX', get_option('prefix'))
+    core_conf.set_quoted('GIO_PREFIX', join_paths(get_option('prefix')))
 endif
 
 configinc = include_directories('.')