libgralloc: fix compiler error with undefined allocSize

Change-Id: I84c5c557b195228d6f40a7aa2a77581566fb5f62
Fix compile error due to singleton changes in MR1

Found here: https://github.com/arco/android_hardware_qcom_display-legacy/commit/a5439a84b59ccf744c1d10ca765aabec5e143cae

Change-Id: Ic822bd7beae95bccf2d7ec84322f469d5c67d394