Fix broken CleanSpec.mk in libs/gui

The CleanSpec is missing parens, which causes $P to be interpreted as a
single variable, and the following printed to the log:

Clean step: rm -rf RODUCT_OUT/obj/SHARED_LIBRARIES/libgui_intermediates

This patch adds parens as needed.

Change-Id: I587998fa67a4884418c286360a577cdbb6ea9a21