games/libretro-mame2003: fix build on GCC architectures

Define __BSD_VISIBLE if it's not defined to uncover u_int:
In file included from src/libretro-deps/libFLAC/cpu.c:83:
/usr/include/sys/sysctl.h:1101:25: error: unknown type name 'u_int'; did you mean 'int'?
1101 | int sysctl(const int *, u_int, void *, size_t *, const void *, size_t);

|                         ^~~~~
|                         int

MFH: 2020Q3 (fix build blanket)