net/ipsumdump: fix build on powerpc64

aggtree.cc:14:24: error: static declaration of '__builtin_bswap32' follows non-static declaration

14 | static inline uint32_t bswap_32(uint32_t u) {
   |                        ^

/usr/include/byteswap.h:38:21: note: expanded from macro 'bswap_32'

38 | #define bswap_32(x) __bswap32(x)
   |                     ^

/usr/include/sys/_endian.h:84:22: note: expanded from macro '__bswap32'

84 | #define __bswap32(x)    __builtin_bswap32(x)
   |                         ^