core: Fix assert printf format string

cppcheck said: core/assert.c:23: warning (invalidPrintfArgType_sint): %d in
format string (no. 2) requires 'int' but the argument type is 'unsigned
int'.

core/assert.c

@@ -20,7 +20,7 @@
 #ifdef DEBUG_ASSERT_VERBOSE
 NORETURN void _assert_failure(const char *file, unsigned line)
 {
-    printf("%s:%d => ", file, line); \
+    printf("%s:%u => ", file, line); \
     core_panic(PANIC_ASSERT_FAIL, assert_crash_message); \
 }
 #else