build: Fix strndup detection on MinGW
GCC and meson conspire together to incorrectly detect that strndup() exists on MinGW as __builtin_strndup, when no such function exists. As a work around, meson will skip looking for __builtin functions if an '#include' is in the prefix, so add '#include <string.h>' when looking for strndup().
See: https://github.com/mesonbuild/meson/issues/3672
Signed-off-by: Joshua Watt JPEWhacker@gmail.com