Add references to eglmesaext.h for build to succeed.
This patch should not be necessary in the future as these
have been added to eglext.h.   
See https://github.com/KhronosGroup/EGL-Registry/commit/1343ecc9f55d3e8ded6a42d7ccfd33391a1dd35f

From: Brian Woodcox <bw@InSkyData.com>
Date: March 18, 2020

--- a/cogl/cogl/meson.build
+++ b/cogl/cogl/meson.build
@@ -48,7 +48,7 @@
 built_headers += [cogl_gl_header_h]
 
 if have_egl
-  cogl_egl_includes_string = '#include <EGL/egl.h>\n#include <EGL/eglext.h>'
+  cogl_egl_includes_string = '#include <EGL/egl.h>\n#include <EGL/eglext.h>\n#include <EGL/eglmesaext.h>'
 else
   cogl_egl_includes_string = ''
 endif

--- a/src/backends/meta-egl-ext.h
+++ b/src/backends/meta-egl-ext.h
@@ -29,6 +29,7 @@
 
 #include <EGL/egl.h>
 #include <EGL/eglext.h>
+#include <EGL/eglmesaext.h>
 
 /*
  * This is a little different to the tests shipped with EGL implementations,

--- a/src/backends/meta-egl.c
+++ b/src/backends/meta-egl.c
@@ -26,6 +26,7 @@
 
 #include <EGL/egl.h>
 #include <EGL/eglext.h>
+#include <EGL/eglmesaext.h>
 #include <gio/gio.h>
 #include <glib.h>
 #include <glib-object.h>

--- a/src/backends/meta-egl.h
+++ b/src/backends/meta-egl.h
@@ -27,6 +27,7 @@
 
 #include <EGL/egl.h>
 #include <EGL/eglext.h>
+#include <EGL/eglmesaext.h>
 #include <glib-object.h>
 
 #define META_EGL_ERROR meta_egl_error_quark ()