android: add virgl to be a valid driver

To avoid the warning:

external/libdrm/libkms/Android.mk:17: invalid GPU drivers: virgl

Signed-off-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/libkms/Android.mk b/libkms/Android.mk
index 4f5e3d2..a406782 100644
--- a/libkms/Android.mk
+++ b/libkms/Android.mk
@@ -3,12 +3,14 @@
 intel_drivers := i915 i965 i915g ilo
 radeon_drivers := r300g r600g radeonsi
 nouveau_drivers := nouveau
+virgl_drivers := virgl
 vmwgfx_drivers := vmwgfx
 
 valid_drivers := \
 	$(intel_drivers) \
 	$(radeon_drivers) \
 	$(nouveau_drivers) \
+	$(virgl_drivers) \
 	$(vmwgfx_drivers)
 
 # warn about invalid drivers