Reduce warnings/errors in libpayload when using picky compiler options

The new build system uses quite a few more -W flags for the compiler by
default than the old one. And that's for the better.

Change-Id: Ia8e3d28fb35c56760c2bd0983046c7067e8c5dd6
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/72
Tested-by: build bot (Jenkins)
Reviewed-by: Uwe Hermann <uwe@hermann-uwe.de>

payloads/libpayload/arch/powerpc/main.c

@@ -41,6 +41,7 @@ char *main_argv[MAX_ARGC_COUNT];
  * This is our C entry function - set up the system
  * and jump into the payload entry point.
  */
+void start_main(void);
 void start_main(void)
 {
 	extern int main(int argc, char **argv);