Explicitly depend on librt on Linux
Explicitly depend on librt on Linux

Some compilation environment already does the right thing, but
some other would fail to find clock_gettime(). This commit makes the
dependency explicit.

Change-Id: I86d1d0e6d62e7dfdb32b58a6d28b65e9db5245cd

realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core