audio/synthpod-lv2: fix build on aarch64

../mapper.lv2/mapper.lv2/mum.h:113:3: warning: implicit declaration of function 'asm' is invalid in C99 [-Wimplicit-function-declaration]

asm ("umulh %0, %1, %2" : "=r" (hi) : "r" (v), "r" (p));
^

../mapper.lv2/mapper.lv2/mum.h:113:27: error: expected ')'

asm ("umulh %0, %1, %2" : "=r" (hi) : "r" (v), "r" (p));
                        ^

../mapper.lv2/mapper.lv2/mum.h:113:7: note: to match this '('

asm ("umulh %0, %1, %2" : "=r" (hi) : "r" (v), "r" (p));

reported upstream: https://github.com/OpenMusicKontrollers/synthpod/pull/54

Approved by: portmgr (tier-2 blanket)