Check the return value of setuid(2).

I'd like to mark setuid() WARN_UNUSED_RESULT to prevent security bugs.
This line would break because the file is compiled with -Werror.

Bug: 34251640
Test: Build, run unit test.
Change-Id: Ibb6fa5e5b899d47f77f2a32da762165de310101d

liblog/tests/liblog_test.cpp

@@ -1831,7 +1831,7 @@ TEST(liblog, __security_buffer) {
         return;
     }
 
-    setuid(AID_SYSTEM); // only one that can read security buffer
+    EXPECT_EQ(0, setuid(AID_SYSTEM)); // only one that can read security buffer
 
     pid_t pid = getpid();