mips_m4k: common_magic should be unsigned

For all architectures we use distinct common magic number,
and this should be a uint32_t type.
Otherwise, comparison with macros will yield compilation
warning.

src/target/mips_m4k.h

@@ -31,7 +31,7 @@ struct target;
 
 struct mips_m4k_common
 {
-	int common_magic;
+	uint32_t common_magic;
 	bool is_pic32mx;
 	struct mips32_common mips32;
 };