Fix a couple sign-comparison warnings.

Bug: http://b/26523949
Change-Id: Id33146b5544147945fee3f14f7f72f39bae449f6
diff --git a/sysdeps_win32_test.cpp b/sysdeps_win32_test.cpp
index 81923cb..1d40281 100755
--- a/sysdeps_win32_test.cpp
+++ b/sysdeps_win32_test.cpp
@@ -66,7 +66,7 @@
     const char* path_val = adb_getenv("PATH");
     EXPECT_NE(nullptr, path_val);
     if (path_val != nullptr) {
-        EXPECT_GT(strlen(path_val), 0);
+        EXPECT_GT(strlen(path_val), 0U);
     }
 }