base: make boot_clock work on host linux, hide it on non-linux.

boot_clock was previously returning zero on any platform that doesn't
define __ANDROID__, including host bionic. Instead of returning a bogus
value, just hide it on non-Linux platforms.

Bug: http://b/37758947
Test: libbase_test32/64 on linux
Change-Id: I96e1d8b92dc44c6308408900cf0d27e1e7db5569
diff --git a/chrono_utils_test.cpp b/chrono_utils_test.cpp
index 057132d..1f1ce15 100644
--- a/chrono_utils_test.cpp
+++ b/chrono_utils_test.cpp
@@ -16,6 +16,7 @@
 
 #include "android-base/chrono_utils.h"
 
+#include <err.h>
 #include <time.h>
 
 #include <chrono>
@@ -25,9 +26,12 @@
 namespace android {
 namespace base {
 
+#if defined(__linux__)
 std::chrono::seconds GetBootTimeSeconds() {
   struct timespec now;
-  clock_gettime(CLOCK_BOOTTIME, &now);
+  if (clock_gettime(CLOCK_BOOTTIME, &now) != 0) {
+    err(1, "clock_gettime failed");
+  }
 
   auto now_tp = boot_clock::time_point(std::chrono::seconds(now.tv_sec) +
                                        std::chrono::nanoseconds(now.tv_nsec));
@@ -41,6 +45,7 @@
       std::chrono::duration_cast<std::chrono::seconds>(boot_clock::now().time_since_epoch());
   EXPECT_EQ(now, boot_seconds);
 }
+#endif  // defined(__linux__)
 
 }  // namespace base
-}  // namespace android
\ No newline at end of file
+}  // namespace android