devel/simavr: fix build on FreeBSD 15.1

Starting with FreeBSD 15.1, we define the macro strdupa() in <string.h>.
The software defines its own copy. Respect its wishes and #undef our
macro to fix the build.

MFH: 2026Q2
(cherry picked from commit ac442b50ab68222476d652af9802a64eef71fd33)