package/gdb: fix shared-only build

The shared build only of gdb is actually not supported.

When --disable-static is given to the gdb's main configure script,
it also disable the static build of bundled libraries like bfd, readline
and opcodes, which must be build statically.

Force the static build of bundled libraries by adding --enable-static in
GDB_CONF_OPTS.

Fixes:
http://autobuild.buildroot.net/results/219/21979e730dca1fce5bdda9a4c7fad4485e788866/
http://autobuild.buildroot.net/results/c10/c1096d0bd22de5c6feba848f743601ad0416a944/
http://autobuild.buildroot.net/results/d8a/d8a5bcc7fa374fb0c916a9d0f33ef283109cb404/
http://autobuild.buildroot.net/results/22a/22a86d0f1df0fc4698c0f734f3d659e6317404a4/
http://autobuild.buildroot.net/results/404/404e61e5c30040ee5756f4b5839149dca38660d4/



And many more.

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