xen/build: Fix MAP rule when called in isolation

xen/build: Fix MAP rule when called in isolation

Now that `make MAP` might rebuild $(TARGET), it needs removing from
no-dot-config-targets.

Otherwise the build eventually fails with:

    CPP     arch/x86/asm-macros.i
  arch/x86/asm-macros.c:1:10: fatal error: asm/asm-defns.h: No such file or
  directory
      1 | #include <asm/asm-defns.h>
        |          ^~~~~~~~~~~~~~~~~

Fixes: e1e72198213b ("xen/build: Fix dependency for the MAP rule")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>