builder: Link virt-index-validate with gnulib.

[Gnulib tries to replace getopt because of the bug shown below.
However because -lgnu was not used in the Makefile, the replacement
failed. -- RWMJ]

from config.log:

             /* This code dumps core on glibc 2.14.  */
             {
               static char program[] = "program";
               static char w[] = "-W";
               static char dummy[] = "dummy";
               char *argv[] = { program, w, dummy, NULL };
               optind = opterr = 1;
               if (getopt (3, argv, "W;") != 'W')
                 result |= 64;
             }

gdb --quiet --readnow ./conftest -ex r -ex bt -ex detach -ex q
Reading symbols from /usr/src/packages/BUILD/libguestfs-1.26.0/conftest...expanding to full symbols...done.
Starting program: /usr/src/packages/BUILD/libguestfs-1.26.0/conftest
Missing separate debuginfo for /lib64/ld-linux-x86-64.so.2
Try: zypper install -C "debuginfo(build-id)=5d1a12e6f0e95331cc4e39df74ecbd5adb76a1f7"
Missing separate debuginfo for /lib64/libc.so.6
Try: zypper install -C "debuginfo(build-id)=a06caa12df8f953a453befa827c3145adaf6269a"
main(323) result 0
main(344) result 0
main(357) result 0
main(370) result 0

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b2dbf8 in _getopt_internal_r () from /lib64/libc.so.6
Detaching from program: /usr/src/packages/BUILD/libguestfs-1.26.0/conftest, process 31042

This change actually fixes build for me on sles11sp3 and 11.4.

(cherry picked from commit effcb99f838da1c4830b4a6b0cc4394b904376f2)