sparc32: fix sparse warning in init_32.c

Fix following warning:
init_32.c:112:22: warning: symbol 'bootmem_init' was not declared. Should it be static?

Fix by adding a proper prototype in pgtable_32.h and drop
the local prototype in srmmu.c

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>