Compile the virtual backend with older Mesa

Summary:
BUG: 403326
FIXED-IN: 5.15.0

Test Plan: N/A

Reviewers: #kwin, fvogt, davidedmundson

Reviewed By: #kwin, fvogt, davidedmundson

Subscribers: kwin, fvogt

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18353

plugins/platforms/virtual/egl_gbm_backend.cpp

@@ -30,6 +30,10 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 // Qt
 #include <QOpenGLContext>
 
+#ifndef EGL_PLATFORM_SURFACELESS_MESA
+#define EGL_PLATFORM_SURFACELESS_MESA 0x31DD
+#endif
+
 namespace KWin
 {