package/grpc: fix build on ubuntu gcc 4.8

package/grpc: fix build on ubuntu gcc 4.8

gcc 4.8 on ubuntu 14.04 does some broken optimization at link-time
which causes grpc to create a grpc_cpp_plugin which quits because
of a failing assertion. The created plugin is itself used during
compilation which lets the build fail.
With the added -Wl,--no-as-needed flag the LTO is disabled and grpc
compiles successfully.

fixes:
- http://autobuild.buildroot.net/results/b554f6f2fb66892273f7520ad6e36923557b229e
- http://autobuild.buildroot.net/results/3ebb2880b9b3fd5154979016391dde897e2c039c
- http://autobuild.buildroot.net/results/c2078e821e0728fe980be2c849c25d82e791a4c2

Signed-off-by: Michael Nosthoff <buildroot@heine.tech>
[yann.morin.1998@free.fr: rewrap the comment]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>