net-mgmt/pmacct: fix build on big-endian architectures
- pmacct.h redefines the system byte-order macros, which fails
because of -Werror=macro-redefined.
- POWER passes configure's unaligned test, but then Assign16(a,b) a=b
truncates a 16-bit ethertype into a 1-byte lvalue. Force byte-copy
Assign macros to fix it.