eliminate a compiler warning: signed/unsigned length

Change-Id: I6139a1b3f9caa8fe3492e4850dcc58d8d5ff0cfb
diff --git a/adb/usb_vendors.c b/adb/usb_vendors.c
index 6d85fb1..e9e14da 100644
--- a/adb/usb_vendors.c
+++ b/adb/usb_vendors.c
@@ -151,7 +151,7 @@
 /* builds the path to the adb vendor id file. returns 0 if success */
 int build_path(char* buff, size_t len, const char* format, const char* home)
 {
-    if (snprintf(buff, len, format, home, ANDROID_PATH, ANDROID_ADB_INI) >= len) {
+    if (snprintf(buff, len, format, home, ANDROID_PATH, ANDROID_ADB_INI) >= (signed)len) {
         return 1;
     }