mips: decompress: fix add missing prototypes

Patch series "mips: address -Wmissing-prototypes warnings".

Address the -Wmissing-prototypes warnings that showed up in mips as the
last major architecture after my patch to enable the option everywhere.


This patch (of 20):

The mips decompressor has some string functions defined locally that are
not declared in the right place:

arch/mips/boot/compressed/dbg.c:12:13: error: no previous prototype for 'putc' [-Werror=missing-prototypes]
arch/mips/boot/compressed/dbg.c:16:6: error: no previous prototype for 'puts' [-Werror=missing-prototypes]
arch/mips/boot/compressed/dbg.c:26:6: error: no previous prototype for 'puthex' [-Werror=missing-prototypes]
arch/mips/boot/compressed/string.c:11:7: error: no previous prototype for 'memcpy' [-Werror=missing-prototypes]
arch/mips/boot/compressed/string.c:22:7: error: no previous prototype for 'memset' [-Werror=missing-prototypes]
arch/mips/boot/compressed/string.c:32:15: error: no previous prototype for 'memmove' [-Werror=missing-prototypes]
arch/mips/boot/compressed/decompress.c:43:6: error: no previous prototype for 'error' [-Werror=missing-prototypes]
arch/mips/boot/compressed/decompress.c:91:6: error: no previous prototype for 'decompress_kernel' [-Werror=missing-prototypes]

Include the string.h header where needed and add a decompress.h header to
have shared prototypes for the rest.

Link: https://lkml.kernel.org/r/20231204115710.2247097-1-arnd@kernel.org
Link: https://lkml.kernel.org/r/20231204115710.2247097-2-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Rothwell <sfr@rothwell.id.au>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>