meson: Restrict gdbstub to user/system builds

Limited builds (tools, documentation) don't need to generate /
build gdbstub files. Only process the gdbstub/ folder when
user / system emulation / acceleration is built.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260310232045.58440-6-philmd@linaro.org
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

meson.build

@@ -3754,7 +3754,6 @@ subdir('qom')
 subdir('authz')
 subdir('crypto')
 subdir('ui')
-subdir('gdbstub')
 subdir('semihosting')
 subdir('audio')
 subdir('io')
@@ -3805,6 +3804,7 @@ if have_system or have_user
                          output: 'decode-@BASENAME@.c.inc',
                          arguments: ['@INPUT@', '@EXTRA_ARGS@', '-o', '@OUTPUT@'])
   subdir('libdecnumber')
+  subdir('gdbstub')
   subdir('target')
 endif