piglit: fix build failure with gold linker

piglit: fix build failure with gold linker

When we use gold linker while DISTRO set to "nodistro", piglit build
fails with the following error:

| ../../../../lib/libpiglitutil_gl.so.0: error: undefined reference to
'dlsym'
| ../../../../lib/libpiglitutil_gl.so.0: error: undefined reference to
'dlerror'
| ../../../../lib/libpiglitutil_gl.so.0: error: undefined reference to
'dlopen'
| collect2: error: ld returned 1 exit status

Fix it by providing '-ldl' to LDFLAGS.

[YOCTO #9851]

Signed-off-by: Maxin B. John <maxin.john@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>