package/gdb: fix build of gdb on riscv

package/gdb: fix build of gdb on riscv

Build of gdb on riscv without host-gdb is broken since commit
4ecd247ead22a6cfb87a4ffafc4be05201328aef because BR2_GDB_VERSION_10 is
never defined if BR2_PACKAGE_HOST_GDB is not selected resulting in the
following build failure:

/bin/bash: line 0: cd: /tmp/instance-0/output-1/build/gdb-10.1/gdb/gdbserver: No such file or directory

So add a BR2_PACKAGE_GDB_TOPLEVEL hidden option as suggested by Thomas
Petazzoni.

Fixes:
 - http://autobuild.buildroot.org/results/ce47d616ee79d5f735779570ebc3b4a9c0f64c6a

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>