Commit f9ced08 switches V8 on Linux over from gettimeofday() to
clock_getres() and clock_gettime(). As of glibc 2.17, those functions
live in libc. For older versions, we need to pull them in from librt.
Fixes the following link-time error;
Release/obj.target/deps/v8/tools/gyp/libv8_base.a(platform-posix.o):
In function `v8::internal::OS::Ticks()':
platform-posix.cc:(.text+0x93c):
undefined reference to `clock_gettime'
platform-posix.cc:(.text+0x989):
undefined reference to `clock_getres'
Fixes#7514.
Signed-off-by: Fedor Indutny <fedor@indutny.com>