x86/alternative: Add a __alt_reloc_selftest() prototype

The newly introduced selftest function causes a warning when -Wmissing-prototypes
is enabled:

  arch/x86/kernel/alternative.c:1461:32: error: no previous prototype for '__alt_reloc_selftest' [-Werror=missing-prototypes]

Since it's only used locally, add the prototype directly in front of it.

Fixes: 270a69c4

 ("x86/alternative: Support relocations in alternatives")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230803082619.1369127-6-arnd@kernel.org