binutils: fix static linking

pkg-autotools.mk already handles static/shared linking, so no need to
explictly force it. Worse even, the build fails with BR2_PREFER_STATIC_LINK
as we end up calling libtool with --static when it tries to link the .so,
breaking the build:

../ld: attempted static link of dynamic object
`../opcodes/.libs/libopcodes.so'
collect2: error: ld returned 1 exit status

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>