build: fix dependency file generation with ENFORCE_UNIQUE_SYMBOLS=y

build: fix dependency file generation with ENFORCE_UNIQUE_SYMBOLS=y

The recorded file, unless overridden by -MQ (or -MT) is that specified
by -o, which doesn't produce correct dependencies and hence will cause
failure to re-build when included files change.

Fixes: 81ecb38b83b0 ("build: provide option to disambiguate symbol names")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>