[POWERPC] Fix sparse warnings in xmon.c

warning: Using plain integer as NULL pointer
warning: Using plain integer as NULL pointer
warning: symbol 'excprint' was not declared. Should it be static?
warning: symbol 'prregs' was not declared. Should it be static?
warning: symbol 'cacheflush' was not declared. Should it be static?
warning: symbol 'read_spr' was not declared. Should it be static?
warning: symbol 'write_spr' was not declared. Should it be static?
warning: symbol 'super_regs' was not declared. Should it be static?
warning: symbol 'mread' was not declared. Should it be static?
warning: symbol 'mwrite' was not declared. Should it be static?
warning: symbol 'byterev' was not declared. Should it be static?
warning: symbol 'memex' was not declared. Should it be static?
warning: symbol 'bsesc' was not declared. Should it be static?
warning: symbol 'dump' was not declared. Should it be static?
warning: symbol 'prdump' was not declared. Should it be static?
warning: symbol 'generic_inst_dump' was not declared. Should it be static?
warning: symbol 'ppc_inst_dump' was not declared. Should it be static?
warning: symbol 'memops' was not declared. Should it be static?
warning: symbol 'memdiffs' was not declared. Should it be static?
warning: symbol 'memlocate' was not declared. Should it be static?
warning: symbol 'memzcan' was not declared. Should it be static?
warning: symbol 'proccall' was not declared. Should it be static?
warning: symbol 'scannl' was not declared. Should it be static?
warning: symbol 'hexdigit' was not declared. Should it be static?
warning: symbol 'flush_input' was not declared. Should it be static?
warning: symbol 'inchar' was not declared. Should it be static?
warning: symbol 'take_input' was not declared. Should it be static?
warning: symbol 'xmon_init' was not declared. Should it be static?

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>