xen: fix header guard generation for asm-generic headers

Dashes were wrongly not translated into underscores, thus generating
an unexpected guard identifier.

Fixes: ee79f378311b ("xen: add header guards to generated asm generic headers")
Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>