Add missing C func to link

lvgl-sys/string.c

@@ -66,3 +66,18 @@ char *strcpy(char *dest, const char *src)
         /* nothing */;
     return tmp;
 }
+
+int strcmp(const char *cs, const char *ct)
+{
+    unsigned char c1, c2;
+
+    while (1) {
+        c1 = *cs++;
+        c2 = *ct++;
+        if (c1 != c2)
+            return c1 < c2 ? -1 : 1;
+        if (!c1)
+            break;
+    }
+    return 0;
+}