x86/purgatory: Include header for warn() declaration

The purgatory code uses parts of the decompressor and provides its own
warn() function, but has to include the corresponding header file to
avoid a -Wmissing-prototypes warning.

It turns out that this function prototype actually differs from the
declaration, so change it to get a constant pointer in the declaration
and the other definition as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230803082619.1369127-5-arnd@kernel.org