net/mpich: fix build on powerpc64

Neither BYTE_ORDER nor BIG_ENDIAN are defined:
In file included from src/mpi/datatype/typerep/dataloop/looputil.c:10:
./src/mpi/datatype/typerep/dataloop/looputil.h:57:2: error: This code assumes that BYTE_ORDER and BIG_ENDIAN are defined
#error This code assumes that BYTE_ORDER and BIG_ENDIAN are defined