mips: DEC: honor CONFIG_MIPS_FP_SUPPORT=n

Include the DECstation interrupt handler in opting out of
FPU support.

Fixes a linker error:

mips-linux-ld: arch/mips/dec/int-handler.o: in function `fpu':
(.text+0x148): undefined reference to `handle_fpe_int'

Fixes: 183b40f9 ("MIPS: Allow FP support to be disabled")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Maciej W. Rozycki <macro@orcam.me.uk>
Cc: linux-mips@vger.kernel.org
Acked-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>