libpayload: Align main() data types

One of many steps to compile with -Wconversion, as unsigned int and int
aren't the same thing.

BUG=b:111443775
BRANCH=none
TEST=make junit.xml shows fewer warnings with -Wconversion enabled

Change-Id: I9673ca70da32a1e5117b27fa89167e03379af9c1
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32183
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

payloads/libpayload/arch/mips/main.c

@@ -17,7 +17,7 @@
 #include <libpayload.h>
 
 /* The argc value to pass to main() */
-unsigned int main_argc;
+int main_argc;
 /* The argv value to pass to main() */
 char *main_argv[MAX_ARGC_COUNT];