[1.22] Fix building against libglvnd-provided EGL headers
Add missing EGL/eglmesaext.h include. When building against mesa-provided headers, this header is included indirectly via EGL/eglext.h.
Also fix typo COGL_EGL_INCLUDE -> COGL_EGL_INCLUDES
Originally !17 (merged) by @puleglot, but that MR is against master, which is apparently dead.
We've been using this in Debian since 24 Feb to fix a build failure with updated glvnd/Mesa packaging, so I can confirm that it has the desired effect.
Edited by Simon McVittie