www/iridium: unbreak with libglvnd (like r545308)

[188/188] LINK gn
ERROR at //build/config/linux/pkg_config.gni:103:17: Script returned non-zero exit code.

pkgresult = exec_script(pkg_config_script, args, "value")
            ^----------

Package gbm was not found in the pkg-config search path.
Perhaps you should add the directory containing `gbm.pc'
to the PKG_CONFIG_PATH environment variable
Package 'gbm', required by 'virtual:world', not found
Could not run pkg-config.

See //third_party/minigbm/BUILD.gn:122:3: whence it was called.

pkg_config("libgbm") {
^---------------------

See //ui/gfx/linux/BUILD.gn:41:5: which caused the file to be included.

"//third_party/minigbm",
^----------------------