package/mysql: do not force a static build for internal programs

mysql wants to statically link some of the programs it uses internally,
like conf_to_src (whatever it is used for) which links against -lz.

This does not work for a shared-only build, as we would be missing
libz.a in that case.

Fix that by not forcing static builds for internal programs.

Fixes a slew of autobuild failures:
    http://autobuild.buildroot.net/results/018/018a579f37c9c8eca5a36fc76e93508dcf9bba35/
    http://autobuild.buildroot.net/results/2cc/2cc481b0ae43713a6fc50a1ca720f91384892af7/
    http://autobuild.buildroot.net/results/d98/d985c5824f9012374d64e6085d6c8ec9b3cd932b/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>