Remove a gcc-4.5 compilation warning:

Remove a gcc-4.5 compilation warning:
bionic/libc/include/../include/stdlib.h: In function 'int grantpt(int)':
bionic/libc/include/../include/stdlib.h:138:23: warning: parameter '__fd' set but not used [-Wunused-but-set-parameter]

By adding __attribute((unused)) to __fd, the warning is gone, and this
attribute is compatible with gcc-4.4.0, gcc-4.3.1 gcc-4.2.1.
There is no any side effect.

Change-Id: I385f0f4da1013ffd1499e391eac9123aafe1f7a5