ARM: 9310/1: xip-kernel: add __inflate_kernel_data prototype

The kernel .data decompression is called from assembler, so it does
not need a prototype, but adding one avoids this W=1 warning:

arch/arm/kernel/head-inflate-data.c:35:12: error: no previous prototype for '__inflate_kernel_data' [-Werror=missing-prototypes]

The same file contains a few extern declarations for assembler
symbols, move those into the header as well for consistency.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>