desktop/mesa.bst: Install a pkgconfig file for glesv2.

The build of new versions of Mesa does not seem to install pkgconfig
file for glesv2 anymore when using libglvnd. It was probably an
oversight. Mesa still installs the headers. Only the library is
provided by libglvnd, and it does not provide a pkgconfig file either.
So for the moment we just generate one manually.
8 jobs for valentindavid/gles2-pkg-config in 140 minutes and 20 seconds (queued for 6 seconds)
latest
Status Job ID Name Coverage
  Flatpak
canceled #248736945
aarch64
app_aarch64

02:20:16

canceled #248736948
armhf
app_arm

02:20:20

canceled #248736942
cache_i686 x86_64
app_i686

02:20:19

canceled #248736931
cache_x86_64 x86_64
app_x86_64

02:20:19

 
  Vm
canceled #248736951
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64

canceled #248736953
armhf
minimal_systemd_vm_arm

canceled #248736950
cache_i686 x86_64
minimal_systemd_vm_i686

canceled #248736949
cache_x86_64 x86_64
minimal_systemd_vm_x86_64