syslinux: add missing dependency on host-util-linux

syslinux: add missing dependency on host-util-linux

If util-linux is not installed system-wide on the host, the build
fails with:

/usr/bin/gcc -Wp,-MT,isohybrid.o,-MMD,./.isohybrid.o.d -O2 -I/home/thomas/projets/buildroot/output/host/usr/include -W -Wall -Wstrict-prototypes  -Os -fomit-frame-pointer -D_FILE_OFFSET_BITS=64 -I/home/thomas/projets/buildroot/output/build/syslinux-6.03/utils -c -o isohybrid.o /home/thomas/projets/buildroot/output/build/syslinux-6.03/utils/isohybrid.c
/home/thomas/projets/buildroot/output/build/syslinux-6.03/utils/isohybrid.c:40:23: fatal error: uuid/uuid.h: No such file or directory
 #include <uuid/uuid.h>
                       ^
compilation terminated.

Therefore, this commit adds a dependency on host-util-linux, which
will ensure that libuuid is available. The resulting isohybrid tool is
really installed, and linked with libuuid:

$ readelf -d output/host/usr/bin/isohybrid

Dynamic section at offset 0x3e00 contains 26 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libuuid.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000f (RPATH)              Library rpath: [/home/thomas/projets/buildroot/output/host/usr/lib]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>