mips: zboot: Fix "no previous prototype" build warning

Without having the putc() prototype forwardly declared the next
build-warning is printed:

arch/mips/boot/compressed/uart-prom.c:6:6: warning: no previous prototype for ‘putc’ [-Wmissing-prototypes]
...

Fix that by adding the "decompress.h" header file with the putc() function
declaration to the source files which have the method redefined.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>