Add extern "C" {...} to linenoise.h to fix the linker error.

That fixes the error:
ninja: Entering directory `out/Default/'
[2/2] LINK example_cpp
FAILED: clang++  -o example_cpp -Wl,--start-group obj/example_cpp.example_cpp.o obj/liblinenoise.a  -Wl,--end-group
obj/example_cpp.example_cpp.o: In function `main':
../../example_cpp.cc:(.text+0x18): undefined reference to `linenoise(char const*)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

Thanks Ami for helping to fix this:
https://groups.google.com/d/topic/gyp-developer/mq-NR67ME9U/discussion

Signed-off-by: Thiago Farina <tfarina@chromium.org>