package/gdb: add optional support for xz

package/gdb: add optional support for xz

When xz was compiled before, gdb will use it as optional dependency:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/bin/gdb | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libdl.so.1]
 0x00000001 (NEEDED)                     Shared library: [libncurses.so.5]
 0x00000001 (NEEDED)                     Shared library: [libz.so.1]
 0x00000001 (NEEDED)                     Shared library: [libm.so.1]
 0x00000001 (NEEDED)                     Shared library: [liblzma.so.5]
 0x00000001 (NEEDED)                     Shared library: [libc.so.1]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>