fix compiler error under FORTIFY_SOURCE=2

frameworks/base/libs/androidfw/ObbFile.cpp:340:5: error: ignoring return value of function declared with warn_unused_result attribute [-Werror,-Wunused-result]
    ftruncate(fd, mFooterStart);
    ^~~~~~~~~ ~~~~~~~~~~~~~~~~
1 error generated.
amake: *** [out/host/linux-x86/obj/STATIC_LIBRARIES/libandroidfw_intermediates/ObbFile.o] Error 1

Change-Id: I7080d64e7a830a10b41c1ebf33c60d32b7c8356e