Make the AHardwareBuffer extension compatible with C.

The eglext.h file must be C-compatible, and is included by vendor C
code. C does not support magicking up the 'struct' keyword.

Fixes the change made in I0da1173688cd27b8dd9e907e535100b9a0605290

Bug: 34050596
Test: built, ran demo app using extension
Change-Id: Iaa3028c3ae2087f7368549565a778ce75110d3cb