Building libGLESv2 on Mac currently results in

.../DisplayCGL.mm:9:10: fatal error: 'libANGLE/renderer/gl/CGL/DisplayCGL.h' file not found

Other files in libANGLE/renderer/gl/cgl are using lowercase `cgl`
for #includes. DisplayCGL.mm should do the same.

Change-Id: I03b68041ef3194eb14d65738e4e4fb12711b1b20
Reviewed-on: https://chromium-review.googlesource.com/320155Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>