libcutils_header exports liblog_headers

cutils/log.h includes log/log.h. Therefore, libcutils_headers needs to
export the liblog_headers so that the clients of libcutils don't have to
explicitly include liblog (or liblog_headers).

Bug: 37342627
Test: build sdk_gphone_x86 userdebug with BOARD_VNDK_VERSION=current
Change-Id: I8875855222b4d073fac4a22bff488c9d082d39df