Fix 'BCCscript' build break.

frameworks/compile/libbcc/include/bcc/bcc.h was updated
in change I3f3d31fc. What used to be "struct BCCscript"
is now "class bcc::BCCscript" for C++ code.

Looks like the framework/base header rsScriptC.h is just
trying to forward-declare the same type, so we just need
to make the same change there.

Change-Id: I6f6f9dd7608af01d519c2e26fd9b5e6649f71902