mips: asm-offsets: add missing prototypes

Building with -Werror and W=1 fails entirely because of warnings in
asm-offsets.c:

arch/mips/kernel/asm-offsets.c:26:6: error: no previous prototype for 'output_ptreg_defines' [-Werror=missing-prototypes]
arch/mips/kernel/asm-offsets.c:78:6: error: no previous prototype for 'output_task_defines' [-Werror=missing-prototypes]
arch/mips/kernel/asm-offsets.c:92:6: error: no previous prototype for 'output_thread_info_defines' [-Werror=missing-prototypes]
arch/mips/kernel/asm-offsets.c:108:6: error: no previous prototype for 'output_thread_defines' [-Werror=missing-prototypes]
arch/mips/kernel/asm-offsets.c:136:6: error: no previous prototype for 'output_thread_fpu_defines' [-Werror=missing-prototypes]

Nothing actually calls these functions, so just add prototypes to shut
up the warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>