mpi: Suppress set-but-unused-variables warnings.

* include/types.h (GNUPG_GCC_ATTR_UNUSED): Define for gcc >= 3.5.
* mpi/mpih-div.c (mpihelp_divmod_1, mpihelp_mod_1): Mark dummy as
 unused.
* mpi/mpi-internal.h (UDIV_QRNND_PREINV): Mark _ql as unused.

Due to the use of macros and longlong.h, we use variables which are
only used by some architectures. At least gcc 4.7.2 prints new
warnings about set but not used variables. This patch silences them.