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

This reverts commit ae29339ca138648eeaee68f801feb02d85ee2abf, which broke the darwin build.

Change-Id: Ib33e2dab197532f04944cb12151002e9219b1bb5
diff --git a/base/chrono_utils.cpp b/base/chrono_utils.cpp
index d73b551..5eedf3b 100644
--- a/base/chrono_utils.cpp
+++ b/base/chrono_utils.cpp
@@ -21,14 +21,17 @@
 namespace android {
 namespace base {
 
-#if defined(__linux__)
 boot_clock::time_point boot_clock::now() {
+#ifdef __ANDROID__
   timespec ts;
   clock_gettime(CLOCK_BOOTTIME, &ts);
   return boot_clock::time_point(std::chrono::seconds(ts.tv_sec) +
                                 std::chrono::nanoseconds(ts.tv_nsec));
+#else
+  // Darwin does not support clock_gettime.
+  return boot_clock::time_point();
+#endif  // __ANDROID__
 }
-#endif
 
 }  // namespace base
 }  // namespace android