meson: fix debug build on Android

debug_stack functions are implemented in another file for Android.
Also add backtrace library dependency.

Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-bu: Kristian H. Kristensen <hoegsber@google.com>
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2435>

meson.build

@@ -786,6 +786,9 @@ if with_platform_android
     dependency('hardware'),
     dependency('sync'),
   ]
+  if with_gallium
+    dep_android += dependency('backtrace')
+  endif
   if get_option('platform-sdk-version') >= 26
     dep_android += dependency('nativewindow')
   endif