deps: fix v8 link error with glibc < 2.17

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.

/cc @indutny @ozlatior - managed to reproduce it on a debian squeeze system.  The original patch was not re-exporting the librt dependency to its dependents.