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>

arch/mips/boot/compressed/uart-16550.c

@@ -8,6 +8,8 @@
 
 #include <asm/addrspace.h>
 
+#include "decompress.h"
+
 #if defined(CONFIG_MACH_LOONGSON64) || defined(CONFIG_MIPS_MALTA)
 #define UART_BASE 0x1fd003f8
 #define PORT(offset) (CKSEG1ADDR(UART_BASE) + (offset))

arch/mips/boot/compressed/uart-alchemy.c

 // SPDX-License-Identifier: GPL-2.0
 #include <asm/mach-au1x00/au1000.h>
 
+#include "decompress.h"
+
 void putc(char c)
 {
 	alchemy_uart_putchar(AU1000_UART0_PHYS_ADDR, c);

arch/mips/boot/compressed/uart-prom.c

 // SPDX-License-Identifier: GPL-2.0
 #include <asm/setup.h>
 
+#include "decompress.h"
+
 void putc(char c)
 {
 	prom_putchar(c);