This function can be static, make it so, this fix a warning treated as
error with W=1:

  arch/powerpc/kernel/btext.c:173:5: error: no previous prototype for ‘btext_initialize’ [-Werror=missing-prototypes]
Signed-off-by: NMathieu Malaterre <malat@debian.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>