Fix more sign-compare issues

The one in the `dummyflasher` is a little peculiar. We actually never
knew the type of the `st_size` field in `struct stat`. It happens to
be `signed` in some systems (e.g. DJGPP).

Change-Id: If36ba22606021400b385ea6083eacc7b360c20c5
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/35800
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

libflashrom.c

@@ -139,7 +139,7 @@ const char **flashrom_supported_programmers(void)
  */
 struct flashrom_flashchip_info *flashrom_supported_flash_chips(void)
 {
-	int i = 0;
+	unsigned int i = 0;
 	struct flashrom_flashchip_info *supported_flashchips =
 		malloc(flashchips_size * sizeof(*supported_flashchips));