powerpc/rtas: Remove 'extern' from function declarations in rtas.h

This header occasionally gains new function declarations without the
leading extern in accordance with current style rules. Leaving the
legacy externs in place is making the header more difficult to read
over time because of the inconsistency. Remove them.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
[mpe: Add names to rtas_call() parameters]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231106-rtas-trivial-v1-7-61847655c51f@linux.ibm.com