mesa-gl: Provide virtual/libgbm

We specificially enable building libgbm via packageconfig but then the
PROVIDES is hardcoded in main recipes, we make that change here so when
using userland as graphics driver, we can build wayland apps

Signed-off-by: Khem Raj <raj.khem@gmail.com>

recipes-graphics/mesa/mesa-gl_%.bbappend

@@ -1,4 +1,5 @@
 PACKAGECONFIG_append_rpi = " gbm"
+PROVIDES_append_rpi = " virtual/libgbm"
 
 do_install_append_rpi() {
     if [ "${@bb.utils.contains("MACHINE_FEATURES", "vc4graphics", "1", "0", d)}" = "0" ]; then