blobmsg/ulog: fix format string compiler warnings

blobmsg/ulog: fix format string compiler warnings

Fixes following compiler warnings:

 blobmsg.c:242:39: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
 blobmsg.c:248:23: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
 ulog.c:100:18: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
 ulog.c:112:16: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
 ulog.c:117:20: error: format string is not a string literal [-Werror,-Wformat-nonliteral]

Signed-off-by: Petr Štetiar <ynezz@true.cz>